They look like creatures from outer space, animals from a scene from Star Wars. They are in fact, creatures from inner space – microcosmic bugs that inhabit the body of a healthy human being.

How many different types of creatures live in or on the average human at one time? That all depends.

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There are two types of tiny animals inhabiting the human body: those that are permanent residents, and then a group of nasty parasites that drop by every now and then.

Microbes inhabit every exposed surface of a healthy adult (such as the skin) or areas accessible from the outside (the intestines, from mouth to anus, eyes, ears and airways).

Mind-boggling populations
In his book Life on Man, bacteriologist Theodor Rosebury gives a full account of the micro-organisms living in or on a human. His findings are quite mind-boggling: he counted 80 distinct species living in the mouth alone. He estimated that 10 million individual bacteria live on an average square centimetre of human skin – comparing this to a mall during Christmas shopping.

Throughout the almost two square metres that make up the surface area of a human, the number of bacteria increases in areas like the oily skin that is found on the side of the nose or in a sweaty armpit. Once inside the body, on the surface of the teeth or throat, concentrations of bacteria can increase a thousandfold. These inside surfaces are the most densely populated of the human body.

Areas where there is liquid flow to remove bacteria, such as the tear duct or genito-urinary surfaces, have much lower populations. Rosebury found no microbial life at all in the bladder and lower reaches of the lungs.

  

Bad bugs
Of course, it’s not just bacteria and viruses that make their homes in people. Parasites like lice are perhaps the most common of these bodydwellers. They can crawl from the hair on your head to your armpits or groin and suck blood every three hours. Bed bugs use their jaws like a saw to pierce the skin, then suck blood for up to 12 minutes before dropping off, leaving a painful bite that takes days to heal.

But they are more itchy than dangerous. The real menaces come in the form of parasites like ticks that can cause nasty and exotic diseases like royal farm disease or Omsk haemorrhagic fever.

Inside your digestive tract you might find, among others, protozoan that cause amoebic dysentery, 20m tapeworms and a hookworm that has a way of finding itself into your bloodstream.

Other creatures that can live in your blood are the hermaphroditic Shistosoma worms, which can lead to a bloody and scarred bladder while the bile-loving Clonorchis sinensis fluke might be floating about in your liver. Perhaps most horrifying of all, the Naegleria fowleri amoeba just loves the warmth that it finds inside your skull, reproducing in its millions until you drop dead.

But not all body parasites creep and crawl – you can find fungi in your hair or yeast-like fungi on your skin that causes athlete's foot, vaginitis or thrush.

Lending a helping hand
The micro-organisms living on your body are not all miniature monsters though. E. coli bacteria have a symbiotic relationship with humans, aiding digestion and maintaining balance in your intestine. Tiny dust mites eat dead skin cells, helping to tidy your home. Some micro-organisms are also little lifesavers. Some types of bacteria naturally produce complex chemicals called antibiotics that are used to treat many diseases.

While it seems that we are being overrun by invisible invaders, all the bacteria on the external surface of a human body together would form an object the size of a pea. The bacteria on the inside would only fill a 300ml container. When you consider the size of a human body, the volume of creatures living on us hardly seems threatening. - Charmaine Horne, Health24

 

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