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- Infectious agents
- Categories of organisms
- Effects of environment on human disease
- Human activity
- Immune response to infection
Occupation and commerce
In occupational and commercial undertakings, people often manipulate their environment and, in so doing, expose themselves to infection. A farmer in his fields is exposed to damp conditions in which disease microorganisms flourish. While clearing out a ditch, he may be infected with leptospires passed into the water in rats’ urine. In his barns he may be exposed to brucellosis if his herd of cattle is infected or to salmonellosis or Q fever. Slaughterhouse workers run similar risks, as do veterinarians. A worker in a dock or tannery may get anthrax from imported hides; an upholsterer may get the disease from wool and hair; and a worker mending sacks that have contained bone meal may contract the disease from germs still clinging to the sack.
Workers in packing plants and shops often are infected from the raw meat that they handle; they are sometimes regarded as carriers and causes of outbreaks of Salmonella food poisoning, but as often as not they are victims rather than causes. Workers in poultry plants can contract salmonellosis, more rarely psittacosis or a viral infection of the eye, from the birds that they handle. Forestry workers who enter a reserve may upset the balance of nature of the area and expose themselves to attack from the undergrowth or the trees by insect vectors of disease that, if undisturbed, would never come into contact with humans. Whenever people manipulate the environment—by herding animals, by importing goods from abroad, by draining a lake, or by laying a pipe through swampy land, and in many other seemingly innocent ways—they run the chance of interfering with microbial life and attracting into their own environment agents of disease that they might not otherwise ever encounter.
The inanimate environment
Dust cannot cause infectious disease unless it contains the living agents of the infection. Yet the term inanimate is a convenient one to use when infectious disease arises from contact with an environment in which there is no obvious direct living contact between the source and the victim of an infection. A pencil is an inanimate object, but if it is sucked by a child with scarlet fever and then by a second child, the organisms causing the disease can be conveyed to the second child. Many such objects—a handkerchief or a towel, for example—may convey infection under favourable conditions, and, when they do so, they are known as fomites.
Dust is perhaps one of the most common inanimate elements capable of conveying disease. Organisms present in dust may get on food and be swallowed, settle on the skin and infect it, or be breathed into the respiratory passages. Some germs—the causative agents of anthrax and tetanus, of Q fever, brucellosis, and psittacosis, for example—can live for long periods dried in dust. Under certain conditions all may be dangerous, but under different conditions there may be no danger. The germ that causes tetanus, Clostridium tetani, is one of the most common germs in dust, but the incidence of tetanus varies greatly in different parts of the world. In many countries it is rare, in others common, and the difference may be related to slight differences in human behaviour or custom. The wearing of shoes in temperate climates protects the wearer against many wounds, while the barefoot child in the tropics sustains many puncture wounds of the feet that, although minor, may yet carry tetanus spores into the tissues of the foot, where conditions may be ideal for germination and the production of toxin. Obstetrical practices in some parts of the world can lead to infection of the newborn child. Other, more subtle influences, such as changes of temperature or humidity, may affect the spread of diseases by dust. A long wet winter followed by a dry summer may encourage the growth of molds in hay, and the dust, when the hay is disturbed, may lead to infection of the farmer’s lungs.
Water is not a favourable medium for the growth and multiplication of microorganisms, and yet many can survive in it long enough to carry infection to humans. Cholera and typhoid fever are both waterborne diseases, and the virus of hepatitis A also can survive in water. But the mere presence of a microorganism in water does not necessarily lead to the spread of disease. People have swum in water polluted with Salmonella typhi without getting typhoid fever, while others eating shellfish from the same water have developed the disease. The same may be true of hepatitis; the eating of clams from infected water has often caused the disease, whereas swimming has not proved to be a hazard. Shellfish concentrate germs in their tissues, which is probably why they can transmit diseases.
Water is carried to humans in pipes for drinking, but it is also carried away in pipes as sewage. Defects in these systems may permit water to pass from one to the other, and microorganisms from sewage may get into drinking water. Wells and other water sources can be contaminated by badly sited septic tanks, manure heaps, or garbage dumps. Properly treated, such water is safe to drink; if drunk untreated, disease may follow.
The soil of gardens and farmlands can harbour disease organisms, as can food. Smoke, fog, and tobacco can affect the human respiratory tract and render it more vulnerable to disease. Air is probably the most common source of infectious agents. The whole of the environment is, in fact, filled with organisms and objects that can transmit infectious agents to humans.