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Man is never alone, for he shares his body with hundreds of millions of organisms most of which are bacteria. More than 600 million individual bacteria live on the skin alone. More than a dozen types of bacteria make their homes on the skin, in the saliva of the mouth and in the lower part of the digestive system. In addition, several fungi, a virus and a type of mite share the body of even a healthy person. The bacteria in the bowel do no harm as long as they stay where they are, and they actually prevent other, disease-causing bacteria (which may be swallowed accidentally) from multiplying. Some bacteria also manufacture vitamin B12 - normally found only in meat - from plant sources. The vitamin helps in the formation of blood cells.
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- Four main groups of bacteria live on human skin and can be found almost anywhere on the body. They are known to scientists as; corynebacteria (including Corynebacterium acnes which causes acne); micrococci; streptococci; and coliform bacteria.
- Other harmless bacteria live in moist areas such as the armpits and groin. They include coagulase negative staphylococci; gram positive cocci; and diphtheroids. The skin of the armpits can harbour up to 800 bacteria per square millimetre (516,000 per sq in), while drier areas such as the forearm have only 20 bacteria per square millimetre (13,000 per sq in).
- Saliva contains at least six types of bacteria, some of which can cause dental plaque, tooth decay and gum disease. They are streptococci; corynebacteria; fusobacteria; Neisseriae; bacilli; and spirochaetes.
- The large bowel is also rich in bacteria. Among the types are: coliform bacteria (especially Escherichia coli); Bacteroides; lactobacilli; closttridia; and streptococci. In the bowel, they are harmless. But if they escape to other parts of the body - as the result of an ulcer, say, or a burst appendix - they can cause peritonitis (an inflammation of the abdominal cavity), blood poisoning and urinary infections.
- Three main types of yeast commonly live on our skin. Malassezia furfur is found mainly on the chest and back. Candida albicans is found mainly between the toes; but it is also found in the mouth, where it can cause thrush, an infection especially common in babies. Trichospora beigelii lives mainly on the scalp. Skin also harbours two other types of fungus (yeasts are themselves fungi) - Trichophyton and Microsporium. They can cause ringworm and athlete's foot.
- The mite Democlex folliculorum, related to the spider family, lives in hair follicles (the cavities containing hair roots) and in the sebaceous (fatty) glands on most adults' faces.
- One strain of the virus Herpes simplex lives inside the nerve fibres of 90 per cent of adults - the only virus to do so. It is usually dormant, but when the host's health is low it can sometimes erupt on the lips as cold sores. The sores can also be brought on by over-exposure to the sun. This strain cannot cause the disease known as genital herpes.
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