Transmission Of Diseases, Prevention of infection And Sewage disposal



Food and drink may become contaminated by bacteria in a variety of ways. If the faeces of a person suffering from an intestinal disease such as cholera or typhoid are deposited in or near a source of drinking water, it is likely that the bacteria present in the faeces will enter the water and be drunk by, perhaps, hundreds of other people. Since it is possible for a person to have these bacteria in his intestine without experienc- ng any symptoms of ill-health, great care should be taken by all individuals to see that their excreta cannot contaminate drinking water.

Disease bacteria may get into food when it is washed in contaminated water or be deposited from the feet of flies which have recently walked on human faeces.

After urinating or defaecating, the hands may be contaminated with urine or faeces so that if the unwashed hands come in contact with food, harmful bacteria from the intestine may be transferred to the food and so infect those who eat it. It has been known for one carrier of typhoid working in a restaurant or canteen to infect hundreds of others.

Droplet infection. The acts of coughing, sneezing, talking or merely breathing, discharge into the atmosphere tiny droplets of moisture from the lungs, trachea, mouth and nose. If a person has a disease of the respiratory tract, such as tuberculosis, these droplets will contain bacteria. The droplets remain suspended for some time in the air, to be breathed in by other people. Crowded, humid, confined situations are very favourable to the spread of disease by droplet infection and unchecked coughing and sneezing propel the droplets over considerable distances.

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Contagion. This is the spread of disease by contact with an infected person as is the case with leprosy. The venereal diseases such as syphilis and gonorrhoea are spread only by sexual contact. The sexual organ of an infected person will carry the spirochaete Treponenma (syphilis), or the gonococcus Neisseria (gonorrhoea). During sexual intercourse the bacteria will be transferred to the sexual organ of the healthy partner and set up an infection. In the case of gonorrhoea, the infection is usually confined to the reproductive organs but syphilis, if untreated, may spread throughout the body producing ulcers, heart disease and mental degeneration. A baby born to a mother with a venereal disease may be infected in the uterus or as it passes through the vagina.

Infection by insects. The possibility of house-flies transmit- ting diseases has already been mentione, There are many biting insects which, if them- selves infected with bacteria or other harmful organisms, may introduce them into the blood stream of Man. Bubonic plague is caused by a bacterium transmitted from infected rats to Man by the bite of fleas which feed on both. The transmission of and malaria (caused by protozoa, not on pp. 190-191.


 The invasion of harmful bacteria is counteract by the bond of healthy mammals in two ways.

(I)The skin forms a defence against the invasion of the blood and body cavities by bacteria. This is effected partly by the physical barrier of the dead, cornified, outer layer of the skin and partly by production of chemicals which destroy bacteria. The eyes, for example, are protected by an enzyme present in normal tears. The lining of the alimentary canal and respiratory passages also resist the entry of bacteria.

(ii) The white cells, leucocytes, engulf the bacteria and secrete chemicals that kill them. Other chemicals, antitoxins, made in the blood, effectively neutralize the poisonous proteins that the bacteria give out. Once an animal has recovered froma bacterial disease, its blood is much better able to combat the bacteria and neutralize its toxins; it is said to have acquired immunity.

Prevention of infection

Food. Most uncooked food contains bacteria, but not usually in sufficient numbers to cause disease, and the lining of the gut and the hydrochloric acid in the stomach will often prevent their development. The techniques of food preservation, involving the destruction of bacteria whose activities would cause the food to decay, also serve to destroy disease bacteria, e.g. typhoid, which could be transmitted by food. Refrigeration does not kill bacteria but prevents their multiplication. The high temperatures of cooking kill most bacteria and their spores, if continued long enough. Bottling and canning kill bacteria by the high temperature used and then prevent their access to the food by sealing it in a vacuum. Poisons such as sulphur dioxide may be added to canned or bottled fruit in a concentration sufficient to kill bacteria but too weak to harm the consumer. Drying, curing, smoking and salting are all methods of food preservation.

In the case of salting and preservation with sugar, the preservation depends on the low osmotic potential of the food which plasmolyses any bacteria which arrive in it.

Food should be eaten soon after it has been cooked or removed from a container and so give invading bacteria, or any not destroyed during cooking, little opportunity to multiply.

Flies, notorious carriers of bacteria, should never be allowed to settle on food, and people who handle food that is to De without subsequ cooking should take great care to be eaten are that their hands do not carry harmful bacteria.

Clean water. To ensure that water is free from harmful bacteria it is necessary (a) to dispose of human sewage in such a way that it cannot contaminate water supplies, and (b) to subject the water to treatment that removes or destroys any harmful bacteria which may be present. These processes are discussed more fully below.

Ventilation. To reduce the chances of droplet infection, buildings need to be adequately ventilated, i.e. provision is made to exchange the air frequently with that from outside so that the suspended droplets with their bacteria and viruses are removed before they reach harmful proportions. In houses this can normally be achieved by windows and ventilators. In public buildings, the normal exchange of air through windows, doors and ventilators may be enhanced by some form of air conditioning which not only exchanges the air but cools it and reduces its humidity.

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Venereal and other contagious diseases. The venereal diseases can only be contracted by sexual contact with an infected person. The symptoms of infection are not necessarily obvious, so casual sexual intercourse with a person whose habits and sexual health are not known could lead to infection. The people most likely to be infected are prostitutes who have sexual relations with a large number of people, some of whom are bound to be infected. Gonorrhoea and, in its early stages, syphilis can be treated by antibiotic drugs but it is vital that , treatment should be sought immediately an infection is nsuspected, and administered by a properly qualified person.

Other contagious diseases such as smallpox (caused by avirus) can be caught by touching almost any part of an infected ll person or his clothing. So even a person sutfering from a mild, contagious skin disease spread in this way should avoid contact with other people as far as possible and ensure that his clothing, as bed clothes and washing materials are kept separate from others. The victim of a serious contagious disease such as small- pox is kept isolated in hospital away from healthy people and his clothing is burned or sterilized.

Control of insects. Flies which may carry disease bacteria on their legs or probosces should be prevented from walking on food for human consumption, especially food which is to be eaten without further cooking. Control of house-flies and insects such as mosquitoes which carry malaria. 

 Personal hygiene. Bacteria and their spores are present all Over the surface of a person's skin. They cannot be totally removed but their numbers can be reduced by regular washing With soap. Thus there is less chance of the person being infected should the skin be damaged. The hands in particular should be washed frequently, especially after using the toilet and before handling food so that intestinal bacteria cannot be taken in with the food. For the same reason, cooking utensils, plates, cups and cutlery should be kept as clean as posible. The bacteria present in an infected wound can cause acute poisoning if they get into food. Therefore, food should not be prepared by anybody with a septic wound, particularly on the hands, unless strict precautions are taken to avoid contamination.

Regular washing of the body and clothing not only keeps down the bacterial population on the skin but also reduces the possibility of harbouring lice and fleas which can transmit serious diseases.

Isolation. People carrying a disease which can be transmitted by touch or droplet infection should not mix with others if the disease is serious and as little as possible if the disease is mild. In particular they should not mingle with crowds where they could pass on the bacteria to a great many people.

Immunization and vaccination. By deliberately infecting a person with a mild form of disease, antibodies are formed in his blood and these will protect him against later infections. 

Antiseptics and antibiotics. Antiseptics are chemicals which act against bacteria. Some antiseptics kill all bacteria and their spores, others kill the bacteria but not the spores and some merely prevent the bacteria from multiplying. Antiseptic chemicals are present in disinfectants which can be used to kill bacteria on the fioors, walls and equipment in buildings such as hospitals where high standards of hygiene are necessary. They can be used to reduce the bacterial population in lavatories, wash-basins and drains. In general, they should not be used on the skin or for treatment of wounds because they may destroy cells of the skin and tissue as well as the bacteria. Thorough washing with clean water is usually adequate for cleansing skin Wounds.

Antibiotics such as penicillin are chemicals extracted Irom bacteria or fungi. They destroy many forms of harmful bacteria and can be used internally against infections of the tissues where they kill the bacteria without harming the cells of the body. For example, an antibiotic like terramycin can be used to control an infection of the middle ear either by the patient's eating the antibiotic or having it injected into the Circulatory system.

Pure water

Compares a shallow well and a deep well. In the former, bacteria from faeces, urine and other organic matter can be washed through the pervious top-soil into the water. In the latter case, surface water cannot reach the deep well because its sides are enclosed in concrete and the water comes from beneath the pervious layer.

In small quantities water can be made safe to drink by boiling it to kill the bacteria or filtering it through special porcelain filters to remove the bacteria. When water is supplied to towns, it is first filtered through beds of sand. The sand particles become covered with a gelatinous film of micro-organisms which feed on the bacteria and so remove them from the water. Any bacteria which escape the filters are killed by exposing the water to chlorine gas before it reaches the storage tanks.

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Sewage disposal

In small isolated communities it is safe to dispose of faeces and urine in deep pits provided that the soil is permeable and there is no source of drinking water within 100 metres to become contaminated by seepage. In a small village or a house with flushing toilets, the sewage can be rendered harmless by letting it run into a septic tank. In the first compartment the solid matter settles to the bottom. In the second chamber a film of protozoa and bacteria growing on the broken bricks traps and digests the suspended organic matter so making it harmless. The sludge has to be removed and buried periodically.

One method of large-scale treatment of sewage from a tow The largest solid particles settle out first and the remaining fluid with fine suspended organic matter is passed through a filter bed packed with coke or broken stones. This is similar to the second compartment of the septic tank. A gelatinous film of protozoa and other micro-organisms grows on the stones, the spaces between the stones allowing circulation of air for the aerobic organisms. The suspended organic particles are eaten and digested by the micro-organisms in the film so that only water and soluble salts escape to the river. The sludge and "humus" are dried and can be used as fertilizer on crops other than those which are eaten without cooking

The water from a sewage works should be free from any bacteria and organic particles but it may contain nitrates and phosphates in solution and so cause problems of eutrophication.


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