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What lies within

Life in the human body

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What lies within

The inside of the human body is alive with life - microscopic life of all kinds. There are at least 200 species of creatures that inhabit a healthy human body. More inhabit the body if it is infested with something.

In his classic work, Life on Man (1969), Theodor Rosebury estimates that there are 80 distinguishable species living in the mouth alone and another 80 species living in the human gut. Among the bacteria living in the mouth are Actinomyces viscosus and Actinomyces naeslundii. Together they constitute the sticky substance known as plaque that must be brushed away so as not to allow it to harden into tartar (calculus).

The number of bacteria present varies with the part of the body. Rosebury estimates that the total number of bacteria excreted each day by an adult ranges from 100bn to 100 trillion.

Where there is a liquid flow, such as the tear ducts or the genito-urinary tract, the number of bacteria is far less. In fact, Rosebury did not find any bacteria living in the bladder and in the lowest reaches of the lungs. Although the microbe density in such places as the human bowel is well into the billions of creatures, they take up little space.

All the bacteria living in the human body would fill a container of only 300 ml (0.63 pints US or 0.53 pints UK). If we are infected, inside the digestive tract can be found the protozoan called Entamoeba histolytica that causes amoebic dysentery and that can burrow through intestinal walls and into the bloodstream to infect the liver, lungs, and brain.

The digestive tract can also play host to hookworms such as Ancylostoma duodenale and Necator americanus measuring about 10 mm (.39 inch) that can also migrate via the bloodstream, a beef tapeworm Taenia saginata that can grow up to 20 metres (65.62 feet) long, and the hermaphroditic Shistosoma worms that can scar the bladder and make it bleed.

The lymphatic system can sustain the Wucheria worm that can be 12 cm (4.72 inches) long. The liver may let stay the bile-loving Clonorchis sinensis fluke. And the human brain can sustain the Naegleria flowleri amoeba. It loves the warmth of the brain and while squatting there continually reproduces in the millions until one day the person drops dead.

Party's over!

Stephen Juan, Ph.D. is an anthropologist at the University of Sydney. Email your Odd Body questions to s.juan@edfac.usyd.edu.au

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