Microorganisms that Cause Disease

PLANTS

Features of plants :

·                Multicellular

·                They carry out photosynthesis as they have chloroplasts.

·                They have cell walls.

·                They cannot move.

Examples :

·                Broad beans

Experiment to show an object is a plant :

Demonstrate one of the following due to photosynthesis

·                leading to evolution of oxygen

·                absorption of carbon dioxide

·                absorption of light

·                starch / sugar production

Differences between plant cell and bacteria :

Plant cell	Bacteria
Nucleus	No nucleus
Chloroplast	No chloroplast
Vacuole	No vacuole
No plasmids	Has plasmids
No flagella	Has flagella
Cellulose cell wall	Slime capsule

ANIMALS

           

Features of animals :

·                Multicellular

·                Can usually move

·                Do not have cell walls

Examples :

·                Humans

·                Rats

·                Fruit flies

Features of insects :

·                3 body segments

·                6 legs

·                Wings

·                2 antennae

·                Spiracles in trachea

·                Compound eyes

FUNGI

Features of Fungi :

·                They feed on dead and decaying organisms by extracellular digestion

·                Do not carry out photosynthesis

Examples of Fungi :

·                Yeast

·                Mucor

·                Mushroom

Structure of Unicellular Fungi, like yeast :Yeast cells have a vacuole, nucleus, cell wall, cytoplasm and cell membrane.

Structure of Multicellular Fungi, like Mucor :

·                Mass of hair – like threads called hyphae

·                The mass itself is called mycelium

·                Fruiting bodies, called sporangia, arise from hyphae

·                Hyphae is not divided into cells

·                Walls of hyphae are made of chitin

           

Nutrition:

·                Heterotrophic

·                Parasitic : They obtain food from other living plants / animals by causing disease to the host. Digested food materials are absorbed by parasitic fungi. In this way, the host is damaged.

·                Saprophytic :Fungi feed on dead organisms. They secrete enzymes and carry out extracellular digestion and absorb the soluble products.

Note :Fungi have no chlorophyll.

Use of Fungi:

·                Mushrooms are edible and are used as human food.

·                Some yeasts are used in bread making. Carbon dioxide produced by yeast acts as the raising agent.

·                Some yeasts respire anaerobically. This is called fermentation and this produces alcohol.

·                Fungi are used in brewing / wine making.

·                Fungi are used to manufacture antibiotics, like penicillin from penicillium, and streptomycinfrom streptomyces, to destroy bacteria. Little harm is done to body cells before the population of bacteria increases.

·                Fungi bring about decay in compost heaps, having dead leaves and dead bodies and they breakdown excreta in sewage. Plantsremove materials from the environment. Fungi replace them. They recycle nutrients in ecosystems by releasing salts from dead organisms. They decompose dead organic materials in the carbon cycle and in the nitrogen cycle.

·                Mycoprotein / single cell protein / Quorn for feeding animals

·                Fungi are used to manufacture vinegar / soy sauce.

·                Fungi are used to flavour cheeses.

·                Fungi synthesise vitamins.

ATHLETE’S FOOT:

Transmission:

·                Direct contact, as in, skin to skin contact

·                Walking barefoot on damp/ wet floors of changing rooms, contaminated with fungal spores

·                Sharing socks / towelscontaminated with fungal spores

·                Skin shed from feet of infected person contains fungal spores that are picked up by uninfected person when walking barefoot. The parasitic fungus establishes itself in the new host.

Prevention of spread:

·                Well ventilated footwear/ sandals are used to inhibit fungal growth.

·                Wash feet thoroughly to remove sweat.

·                Use disinfected footbath at swimming pools.

·                Dry yourself thoroughly between the toesor use antifungalpowder

·                Change socks regularly

·                Fungicidal drugs and creams are used for treatment.

·                Wear slippers in changing rooms

·                Disinfect floors of changing rooms regularly

·                Sufferers should not use swimming pools

·                Don’t share towels, socks, footwear to reduce the chances of cross infection.

·                Use disinfectant in swimming pools

Nutrition of Fungus causing Athlete’s Foot :

·                The fungus is parasitic.

·                It lives in the surface of the body on the damp skin between the toes.

·                It secretes extracellular enzymes to digest skin cells in presence of water.

·                The products of the digested skin are absorbed into the hyphae by diffusion.

THRUSH

Transmission:

·                Contaminated water

·                Unsterilisedfeeding bottles (for babies)

Prevention of Spread :

·                Boil / chlorinate water

·                Sterilise all bottles

If in genital areas,

·                Wear condom

·                Avoid promiscuous relationships

·                Treatment of patients using anti fungal creams / drugs

Why does the spread of malaria differ from that of Athlete’s foot?

·                Athlete’s Footis transmitted by direct skin contact from person to person – no vector is needed

·                Malaria needs a vector – mosquito transmits malaria by bites. Transmission is not directas Plasmodium has to mature in the mosquito.

·                Transmission is not direct as organism lives in blood, not on the surface

VIRUS

Features of Viruses :

·                Very small

·                Can reproduce only in other cells

Examples of Viruses :

·                HIV

·                Tobacco Mosaic Virus

Structure :

·                Viruses are microscopic.

·                They have a protein coat / capsomere / capsid, enclosing a strand of nucleic acid,either DNA or RNA.

·                They attach themselves to host cell membrane.

·                They digest host cell membrane by secreting enzymes and the viral DNA enters the host cell.

·                Viruses reproduce / are active only inside cells of living organisms.

·                They do not carry out all other characteristics of living organisms.

·                They can appear inert / crystalline.

Reproduction:

Viruses can only reproduce inside a living cell.

·                They attach themselves to the host cell membrane.

·                They secrete enzymes to digest host cell membrane and allow viral DNA to enter the cell.

·                The protein coat remains outside the host cell.

·                The viral DNA enters and invadesthe host cell nucleus and reprogrammes host DNA to produce more viral DNA.

·                Host cell is used by the virus to form capsids / protein coats.

·                Each thread acquires new protein coat.

·                The cell membrane breaks down.

·                The viruses burst out of host cell.

·                The host cell is thus destroyed.

POLIOMYELITIS

Treatment:

·                Rest with good nursing

·                Help with breathing is provided by using iron lungs

·                Physiotherapy once muscles are free of pain

·                Walking aids like crutches are used.

·                Treat secondary infections

Transmission:

·                Airborne droplets

·                Houseflies sit on faeces and come into direct contact with food, contaminating it in crowded areas

·                Water borne

Prevention of spread:

·                Educational information is provided, so the public have knowledge of how to prevent spread

·                Salk / Sabbine Vaccine and vaccination programmes for young children / infants

·                Improved living conditions

·                Improved sanitation by water treatment

·                Isolation of / Quarantine sufferers in regions of outbreak, so that spread of virus is greatly reduced.

·                Avoid overcrowding / Poor ventilation

·                High standard of hygiene

Why does it fall?

·                Improved living conditions

·                Better treatment for patients

·                Vaccination

HIV AIDS

Condoms :They prevent sperms from reaching the ovum. They are placed over the penis or line the vaginabefore sexual intercourse, so that the body fluids of the male and the female do not mix during sexual intercourse. HIV Virus is found in body fluids, like semen. Only chance of transmission of HIV is through leakage, due to poor use of condom.

Transmission :

·                Using syringe needles with infected blood

·                Direct transfusion of infected / contaminated blood

·                Sexual transmission as lining of vagina / penis lacks an epidermis

·                Transmission from mother to fetus across placenta

·                Objects that pierce skin may be contaminated by user from user. If it is not unsterilised, the virus can pass to second user

Effect on the body:

·                The HIV Virus destroys the body’s immune system by attacking white blood cells

·                The body is unable to react to antigens.

·                The person is more susceptible to infections

·                Carcinomas develop

·                AIDS develops

Prevention of Spread :

·                Avoid multiple sexual partners as the disease is sexually transmitted. It is better to have a single partner / Don’t have sexual intercourse / don’t be promiscuous

·                Use a condom during sexual intercourse

·                Don’t share needles / razors / toothbrushes as they be contaminated with virus / sterilize needles before use

·                Monitor blood samples at transfusion centres, so that infected blood is not taken. Use HIV free blood for transfusions

·                Secondary infections are treated using antibiotics.

·                Drugs are given to boost immunity

·                Sex education

·                Trace contacts / Screening those at risk

·                HIV positive mothers should not breast feed

·                Caesarian section for HIV positive mothers

Note :Virus cannot penetrate unbrokenhuman skin.

Common Cold :Common cold is caused by viruses. Antibiotics are effective against bacteria only, not against viruses. Overuse of antibiotics can lead to emergence of antibiotic resistant bacteria.

Transmission :Airborne droplets coughed out by the patient.

Prevention of Spread :

·                Use handkerchief

·                Avoid overcrowding

·                Good ventilation

Treatment :

·                Treatment is symptomatic

·                No cure is available

·                Nursing or bed rest

Note :Vaccine is available against polio, but not for a comparatively minor infection, like common cold as it is not cost effective and the virus mutates frequently.

This is spread by airborne droplets, spread by sneezing. So, in more crowded areas there are greater chances of spread of infection.
BACTERIA

Structure : They are very small in size. They have a polysaccharidecell wall, cell membrane, cytoplasm, coil of DNA, flagellum, slime capsule, plasmids and they have no nucleus.

Nutrition :

·                Parasitic :They cause disease to host (living organisms, like plants or animals). Bacteria enter cells of the host and cause loss of nutrients. This damages the host.

·                Saprophytic nutrition, where bacteria secrete extracellular enzymes to digest food externally and absorb the soluble products by diffusion. They feed on dead organic matter

·                Photosynthetic / Autotrophic

Uses of bacteria to humans :

·                Bacteria decompose dead plants and animals and recycle nutrients in an ecosystem. Nutrients are returned to the soil. Humus / compost / manure is formed.

·                Bacteria recycle nutrients. In sewage treatment, anaerobic bacteria produce biogas, used for domestic heating purposes.

·                Food production : yoghurt, cheese flavouring, soy sauce

·                Biotechnological processes : In genetic engineering, bacterial plasmids are used as vectors for gene transfer.

·                Bacteria synthesise vitamins.

Growing Bacteria in the Laboratory :

·                Agar jelly is put on sterilisedPetri dishes.

·                The food sample is put on the agar / A metal loop is used to streak agar with the food sample.

·                It is then incubated at a suitable temperature for a few days.

·                Observe plates for bacterial growth.

·                The agar is stained.

·                Identify bacteria under microscope.

TUBERCULOSIS

Transmission:

·                Airbornedroplets: Patient breathes out droplets during coughing which is inhaled by an uninfected person.

·                Drinking infected and untreated milk

Prevention of spread:

·                Vaccination programmes or provide BCG vaccine

·                Use of early detection methods or screening techniques, likemass X – rays and Mantoux skin test

·                Pasteurisation of milk (TT Cattle)

·                Screening / TT of dairy herds

·                Use of multiple antibiotics, including streptomycin, to reduce the risk of cross infection

·                Isolation of patients

·                Improved living conditions – less overcrowded housing

·                Improved diet

·                Better health education about how the disease spreads

·                Use handkerchief

·                No spitting

Why is it on the rise?

·                People are less careful about obtaining vaccination

·                Antibiotic resistant forms of tuberculosis bacteria emerge.

·                Immigration of infected people into the area.

·                AIDS and Opportunistic infection by Mycobacterium Tuberculosis

·                Breakdown of social conditions, due to wars

·                Rapid travel makes spreading the infection much easier

Treatment :

·                Good diet

·                Rest / no stress

·                Use of antibiotics, like streptomycin to reduce the risk of cross infection

·                Isolation of patient

Why early detection is important?

·                To prevent major lung damage

·                To prevent infection from spreading to other body parts

Why might it become an epidemic?

·                It spreads rapidly directly from person to person through the air

·                It becomes drug resistant

·                No vector / intermediate host are needed

·                No special conditions are needed

TYPHOID

Transmission :

Houseflies

·                Houseflies land on faeces of an infectedperson, that contains the typhoid bacteria.

·                The bacteria stick to the hairy legs of the housefly and on the pads on its feet.

·                Houseflies feed on faeces.

·                They secrete saliva and vomit the contents of their stomach.

·                The digested products are sucked up, including bacteria.

·                Houseflies fly from one food to another.

·                When they alight on food, bacteria is transferred to human food. So, they act as vectors.

·                Humans ingest the typhoid bacilli.

·                Thus, houseflies transmit pathogenic typhoid bacteria to human food from their feet, body, vomit and faeces.

Humans

·                Infectedhumans act as vectors.

·                The pathogenic bacteria is present in faeces.

·                The bacillus is transferred to hands, if hands are not washed after using toilet.

·                It is transferred to food and utensils.

Vector –Houseflies, Cockroaches, Humans

Infection :Bacterial numbers are low at infection. They increase in numbers by reproduction by binary fission. It takes time for their numbers to increase. Higher numbers produce more toxins that cause symptoms to appear.

Ways of keeping houseflies away :

·                Kill adults by insecticides

·                Use fly traps

·                Refuse them from kitchens

·                Wrap all food scraps

·                Cover all foods

·                Proper disposal of refuse by burying to prevent breeding of flies

·                Store food in refrigerators

·                Dispose faeces hygienically

·                Keep plates / cutlery / cups away from flies

·                Sterilise food working surfaces

Prevention of Spread :

·                Control housefly by using insecticides

·                Cover food / Put food in fridges

·                Remove garbage from house / kitchen / cover dustbins to prevent fly breeding / transfer from faeces

·                Sanitary disposal of faeces as bacteria are present in excreta

·                Chlorination / Boiling of drinking water as bacteria are transmitted in water

·                Isolation of patients to prevent transmission by contact with patients and their property

·                Treatment of patients, using antibiotics, like chloromycetin to kill bacteria in patient

·                Wash hands as the person may have contact with infected faeces or vomit and transmit it to food

·                Ban carriers from kitchensas they pose the danger of transferring bacteria to food. If hands are not properly washed, bacillus from hands can contaminate work surface and utensis and contaminate food. If they work at kitchens / hotels, food is served to a large number of guests. Guests could spread infection over a wide area, causing a typhoid epidemic.

·                Wear gloves to handle food

·                Medical checks on catering / hotel staff

How can typhoid spread rapidly in a community?

·                Food in stalls and shops is contaminated.

·                Water is contaminated.

·                Carriers, working with food in public places,pass bacteria with faeces into drinking water / onto plants / shellfish.

·                Chance is greater in largely populated areas where hygiene is poor

·                This is carried by houseflies

Signs and Symptoms :

·                High fever

·                Diarrhoea and vomiting

Effects on the body :

·                High fever denatures enzymes and reduces enzyme activity.

·                This leads to loss of metabolism.

·                Severe vomiting causes dehydration of body cells.

·                This damages body cells.

CHOLERA :

Prevention of spread :

·                Vaccination is provided. The vaccine for cholera contains weakened or dead organisms. Antibodies are formed, which bring about a rapid response on infection.

·                Wash hands to remove bacteria.

·                Boil or chlorinate water before use to kill bacteria.

·                Faeces are disposed hygienically.

·                Do not let flies sit on food.

·                Isolation of patients

·                Infected people are treated with antibiotics, like chloromycetin and tetracycline.

DIPHTHERIA :This is caused by bacterium and is transmitted by airborne droplets.

GONORRHOEAE :

Transmission:

·                Exchange of body fluid during sexual intercourse

·                Transfer from mother to fetus

Prevention of Spread :

·                Reduce the number of sexual partners

·                Avoid prostitutes

·                Use condoms

·                Prompttreatment using antibiotics, for example, penicillin

·                Tracing infected sex partners

·                Health education

·                Abstinence

SALMONELLA / DYSENTERY

Transmission :Borne on meat and vegetables

Prevention of Spread :

·                Wash all fresh food in chlorinated water.

·                Cover all food

·                Keep food scraps / dustbin far from kitchen

·                Wash hands before preparing food

·                Keep food at low temperature

·                Destroy vermin, e.g. cockroach, housefly, rat

·                Wash work surfaces with disinfectant

·                Avoid food past sell by date

·                Wash utensils / tools before use

ANTIBIOTICS :

·                Antibiotics are chemicals, produced by fungi that kill pathogenicbacteria.

·                An example of an antibiotic is penicillin.

·                They prevent reproduction of bacteria.

·                They cause no harm to human cells.

·                The population of pathogens is reduced drastically.

·                This reduces symptoms due to bacterial infection.

·                Patient recovers from life – threatening diseases.

Differences between Fungi and Virus :

Fungi	Virus
It has a cellular structure and has a nucleus	No cellular structure and no nucleus. Only a strand of DNA or RNA
Has cytoplasm	No cytoplasm
Has cell wall / cell membrane	Has only protein coat
Larger in size due to mycelium of hyphae	Small in size
Free living	Function only in living cells by reproduction

Differences between virus and bacteria:

VIRUS	BACTERIA
They are much smaller than bacteria.	They are much larger than viruses.
They cannot be active outside the host cell – they only reproduce inside living cells.	They reproduce outside living cells and are active outside living cells.
They lack cytoplasm.	They have cytoplasm.
They have protein coat and no cell membrane or cell wall.	They have a cell membrane and cell wall, but no protein coat.
No organelles	Few organelles, like mitochondria
They are not destroyed by antibiotics.	They are destroyed by antibiotics.

Differences between bacteria and fungi:

FUNGI	BACTERIA
They have nuclei	They have no nuclei.
They tend to be multicellular / coneocytic	They are unicellular.
They have mitochondria.	They have no double membrane structures, like mitochondria.
Fungi have no plasmids.	Bacteria have plasmids.
Cell wall made of polysaccharide sugars and proteins	Cell wall is made of chitin
No nucleus	Many nuclei
No mycelium of hyphae	A mycelium of hyphae

FOOD PRESERVATION

Dehydration / Drying –

·                Removal of water inactivates microorganisms.

·                Bacteria cannot reproduce.

·                The food cannot decay as no digestion occurs in absence of water.

Salting / Syrup –

·                The osmotic potential of the food is raised.

·                The bacteria lose water by osmosis down concentration gradient.

·                This makes water unavailable to microorganisms.

·                Bacteria / fungi cannot function in limited water, so they are inactivated.

·                They do not reproduce. So, food does not decay.

Refrigeration / Freezing –

·                Temperatures in refrigerators are well below 0°C, such as, in the range – 12°C to – 18°C.

·                Such low temperaturesremove heat from microorganisms.

·                This inactivates bacterial enzymes.

·                So, they can neither reproduce nor metabolise as enzyme activity is inhibited.

·                Thus no decay occurs.

·                Refrigeration restricts access by vectors, such as, flies.

Note :If food is kept at low temperatures, such as – 4°C, bacteria reproduce less rapidly than at higher temperatures. So, build up of toxins / decay is less rapid.

Canning –

·                Food is heated to kill all active bacteria and spores.

·                Food is then put into airtight cans.

·                Food is then sealed to prevent entry of new microorganisms and oxygen.

·                No decay at all.

Vacuum Packing –

·                The microorganisms are killed or inactivated as they lack air (oxygen).

Radiation –

·                Food is sealed in a container.

·                Food is irradiated to kill microorganisms.

Pickling –

·                The pH of the food is changed as food is soaked in vinegar.

·                The low pH is unsuitable for bacterial activity.

·                So, microorganisms are inactivated.

Smoking –

·                Food is suspended over wood smoke for some time.

·                Chemicals in smoke destroy bacteria.

·                This is often combined with dehydration or vacuum packing.

Note :Freezing, salting, canning, smoking and dehydration ONLY are used to preserve meat.

FOOD HYGIENE

·                Food is washed and giblets, guts, feather, feet are removed.

·                Food is covered (until cool to eat) and stored properly to prevent access to houseflies that carry pathogenic bacteria

·                Same knife is not used for raw and cooked meats. Raw meat may contain pathogenic bacteria, which may be transferred to cooked meat and reproduce rapidly, causing food poisoning. Washing removes bacteria from knife.

·                Food is kept in refrigerators (below 4°C)or in cool places to prevent rapid bacterial growth / formation of toxins. It is then defrosted.

·                Food is cooked at high temperatures to destroy bacteria present

·                Wash hands before handling food to prevent contamination of food.

·                Sterilize utensils and work surfaceby washing to prevent contamination of food by bacteria.

·                Public health inspection of premises to prevent contamination of food

Note :

·                Eat food soon after cooking it, before recontamination of food from the air or by flies occur.

·                Food, such as, salad vegetables, must be washed in chlorinated water, as they may carry bacteria, eggs of parasites, pollutants from the soil or from the manure used. They may not have been removed / destroyed by cooking, but are destroyed by chlorine.

Dangers of Poorly Prepared Food:

·                Flies settle on food and bacteria are added to food

·                Bacteria reproduce and produce toxins that spoil food.

·                Bacteria infect humans on eating, causing food poisoning / vomiting / diarrhoea.

Endemic :A disease which is always present in the population of a certain geographical area at low levels / in limited numbers is called an endemic.

Epidemic :Sudden outbreak of a disease with large numbers of people being infected over a wide geographical area at the same time due to rapid spread of the disease is said to be an epidemic. The number of infected cases is much higher than normal levels.

Vector :Vector carries the disease causing organism and transmits it to humans. Humans suffer from the disease. The vector is not affected by the organism / shows no symptoms of disease.

Infection :Metabolism of microorganisms is rapid. They produce waste products / toxins, that cause high fever and cell damage.

Parasite :A living organism that feeds on another living organism. It benefits itself and damages its host.

MALARIA

Transmission :The female Anopheles mosquito is the vector.

·                It bites / pierces human skin and sucks human blood.

·                Protein from human blood are needed to lay eggs.

·                If the victim has malaria, the microscopicmalarial parasite, Plasmodium enters the mosquito with blood.

·                When biting, mosquito injects its saliva to prevent blood clotting.

·                Saliva contains parasite.

·                Parasites mate and mature in the salivary glands of the mosquito.

Life cycle of mosquitoes :

·                Adult mosquitoes fly in the air.

·                Female anopheles mosquito pierces human skin to suck blood.

·                They need protein from human blood to lay eggs.

·                Adults mate.

·                Eggs are laid on stagnant water.

·                Eggs hatch to form larvae.

·                Larvae and pupae hang from water surface to obtain oxygen via breathing tubes.

·                Larvae feed on plankton in water.

·                Pupae develop into adult mosquitoes.

·                Mosquitoes undergo metamorphosis from larvae to pupae to adults.

Prevention of spread :

To kill larvae / pupae:

·                Drain water in ponds as female Anopheles mosquito lays eggs in water and eggs develop into larvae and metamorphose into pupae in water. So, breeding ground is removed.

·                Add oil to water so that eggs are no longer laid. Oil kills larvae and pupae by sinking and drowning them and by preventing gaseous exchange from occurring.

·                Biological control :Insectivorous fish, like Tilapia eat larvae / pupae or parasites, like Bacillus Thuringiensis are added.

·                Insecticide to destroy / kill larvae.

To kill the adult / mated female :

·                Spray with insecticides to reduce the numbers of adult mosquitoes, so that the number of humans bitten / infected falls.

·                Sleep under mosquito nets / screens are used as adult mosquitoes cannot reach humans.

·                Anti malarial drug, quinine is used to kill Plasmodium

·                Removing tins / covering pools as these are used by mosquitoes as breeding grounds.

·                Use sterile males for mating

Mosquito is the vector for Plasmodium.

Adaptations for female Anopheles mosquito as a vector of malaria :

·                Mouth parts for piercing and sucking blood

·                Parasite in blood is transferred to mosquito (host)

·                Female flies from host to host

·                The mosquito injects anticoagulant in saliva while feeding, transferring parasite into blood

·                Mosquitoes breed rapidly

Why is it difficult to control malaria ?

·                No vaccination

·                Mosquito shows resistance to insecticides

·                No drugs to destroy Plasmodium

·                Mosquitoes can lay eggs in small amounts of water

·                It is difficult to remove all breeding areas / kill all mosquitoes as they are large in number

Effect of global warming on malaria :

·                Mosquitoes require warm climate.

·                A larger area of the world becomes warmer and wetter.

·                Mosquitoes will spread to more areas as breeding sites increase, taking malaria with them to a larger area.

·                So, the number of infected cases increases.

SCHISTOSOMIASIS

Causative Agent :Flatworm / blood flukes are parasites that enter the body of their host (humans) through the skin, causing fever and blood in urine.

Snails as Vectors :Schistosoma eggs are present in urine / faeces of infected person. Eggs hatch in water to release larvae. Larva enter the body of snail, feed and develop into second type of larva, that are released by snails into water. This larva can penetrate human skin.

Lifecycle of Schistosome :

·                The adult schistosome lives in bladder and male and female blood flukes mate.

·                Eggs are released along with blood in faeces / urine.

·                They deposit in river / on soil / on plants / in damp places.

·                Eggs release larvae (miracidium) into water.

·                The free living ciliated larva, miracidium swims to snails.

·                Snails act as secondary hosts.

·                Miracidium reproduces many times in the body of the snail.

·                Lots of larvae are set free from snail.

·                The new larvae released is called cercaria and is of another type.

·                Cercariawaits on grass / in waters for humans.

·                Cercariapenetrate skin and enter the blood / circulatory system.

Prevention of Spread :

·                Hygienic disposal of urine / faecesafter proper sewage treatment

·                Use of molluscicides / copper saltsto kill snails so that there is no host for the eggs to develop into the larval stage

·                Drugs to kill blood fluke / flatworms in humans to prevent risk of cross – infection

·                Treatment of drinking water

·                Avoid contact with untreated water

·                Farmers and workers should use footwear / boots which flukes cannot penetrate

·                No bathing in water where snails live

·                Control of weeds

·                Use biological control, like crayfish and ducks to eat snails so that there is no host for the eggs to develop into the larval stage

·                Drain irrigation ditches (when growing season is over) so that there is no water for fluke to swim

·                Replace water canals with piped water

·                Dig wells to supply water so that snails cannot enter water

Why does it not become an epidemic / remain endemic?

·                Schistosomiasis requires water and water to complete the life cycle of blood fluke

·                It is transferred to humans when they come in contact with water contaminated with blood fluke

·                These conditions are only available in certain areas. So, the disease cannot spread outside these areas and develop into a world wide epidemic

TYPHUS

Transmission :

·                This is caused by parasitic rickettsia and is louse – borne.

·                Lice are parasites that live in pubic hair and suck blood from humans (host).

·                Lice are not harmed by rickettsia.

·                They are transmitted by close bodily contact during sexual intercourse.

·                Their eggs are cemented to the base of body hair.

·                Bacterium in the faeces of louse, are left on human skin.

·                Louse bites scalp of hair and causes irritation.

·                Scratching breaks skin surface, allowing entry of pathogens into the body / Typhus rickettsia may be rubbed into wound.

Prevention of Spread :

·                No sexual intercourse

·                Use insecticide lotions

·                Comb out eggs / nits from hair

·                Regular health checks / health education

·                Avoid overcrowding

·                Regular washing of clothing and bedding

·                Do not share combs / brushes / hats

GENERAL COURSE OF A DISEASE

·                Infection : entry of pathogen

·                Incubation period :pathogens reproduce in the body

·                Pathogens release toxins

·                This causes signs and symptoms of disease

RAPID RISE IN HUMAN POPULATION IN SOME REGIONS

·                Overcrowded living quarters, leading to transmission of more airborne diseases, such as, tuberculosis and polio

·                Unhygienic living conditions

·                Sewage treatment facilities are not available, spreading diseases, like, cholera and typhoid

·                Shortage of food and clean drinking water

·                Malnutrition

·                Protein deficiency diseases

·                vitamin deficiency diseases