Life on Man [1] Reviewed in the New Scientist [2]

In his seminal work Life on Man, bacteriologist Theodor Rosebury gives a full biological and historical account of the microbes that live on the average human. The numbers involved are huge -- Rosebury tells us: "If we are to get to the microscopic center of this with our eyes open and our stomachs steady, we might do better to look gingerly and sip instead of gulping . . . the life on man consists of microbes in extraordinary variety and large numbers."

The microorganisms that inhabit the body of a healthy human being are known as the normal microbial fauna, and they come in two different types -- (1) those that are permanently resident and (2) those that are transient. Of course, any number of fascinating and nasty parasites can join this microbial community and make the human body their home.

The figures that he grapples with are quite mind-boggling. For example, he counted 80 distinguishable species living in the mouth alone and estimated that the total number of bacteria excreted each day by an adult to ranges from 100 billion to 100 trillion [Editor's Note: The current estimate for the number of human cells at any given moment in the adult body is only [10 - 15] trillion, including neurons/glial-cells in the brain, but not including a lifetime of sperm ejaculation in males, which would raise the estimate considerably]. From this figure it can be estimated that the microbial density on a square centimeter of human bowel is around 10 billion organisms (1010/cm2) [==> 1.5 x1013 or yielding a total of 15 trillion microbes, based on 2 m2 surface/person].

Microbes inhabit every surface of a healthy adult human that is exposed to the outside, such as the skin, or that is accessible from the outside -- the alimentary canal, from mouth to anus, plus eyes, ears, and the airways.

Rosebury estimates that 50 million individual bacteria live on the average square centimeter (5x107/cm2) of human skin [5x107/cm2 x 20,000 cm2/person = 1011 bacteria], describing the skin surface of our bodies as akin to a "teeming population of people going Christmas shopping."

However, this figure can vary widely throughout the almost two square meters that make up the total surface area of a human (2 m2). In the oily skin that is found on the side of the nose or in a sweaty armpit, the figure can increase tenfold, while once inside the body, on the surface of the teeth, throat or alimentary tract, these concentrations can increase a thousandfold. These inside surfaces are the most densely populated region of the human body.

Conversely, on those surfaces where there is liquid flow removing bacteria, such as the tear duct or genito-urinary surfaces, the populations of organisms are much thinner. Indeed, Rosebury could detect no microbial life at all in the normal bladder and lower reaches of the lungs.

Yet, while the figures appear huge, he estimates that all the bacteria living on the external surface of a human would fit into a medium-sized pea, while all those on the inside would fill a vessel with a capacity of 300 milliliters (300 ml) [the size of a small coffee mug].

These figures increase if disease organisms are present, such as a virus or other infection, but not by any statistically significant amount. While the total number of organisms living on us is huge, when one considers the total volume of the human body, the volume of species using us as home is not really that great.

As to the total number of species that are inhabiting a healthy body, estimates vary as more species are discovered on a seemingly regular basis, but Mark Pallen, a Professor of Microbiology based at the Queen's University of Belfast, reckons that the figure is greater than 200 species.

"There are more than 80 that live in the mouth alone and studies that have been carried out at the Laboratory of the Ecology and Physiology of the Digestive System in Jouy-en-Josas, FRANCE, suggest that at least another 80 live in the gut, with many others living on our skin. It's impossible to be precise, but our permanent resident population certainly exceeds 200 species," he says. "The human genome carries a maximum of 100,000 genes [Editor's Note: Now thought to be a maximum of 50,000 genes], yet the average bacterial genome has 2,000 genes. Therefore, there are actually [4 - 6] times as many genes found in the bacteria that live on humans, as there are in the human genome itself." [Editor's Note: Don't forget that we must subtract the number of common bacterial genes, which are redundant in this calculation.]

Of course, it's not just bacteria and viruses that make people their home. In his books Roger M. Knutson describes the wide range of parasites that live both on and inside you [6, 7]. These tend to be macroscopic organisms, and some of them can be pretty gruesome creatures.

Lice are perhaps the most common of these body dwellers. They have the ability to get everywhere from your hair to your armpits to your groin. Nonetheless, they tend to be more itchy than damaging -- unlike ticks, which can cause any number of nasty and exotic diseases from Royal Farm Virus to Omsk Hemorrhagic Fever. [Editor's Note: And don't forget Lyme Disease.] And then there is the scabies mite, which is believed to infest millions of humans worldwide, and is able to burrow into the body to hide itself, causing a nasty itch.

Fortunately, its close relative, the follicle mite, which is found on everybody in the world, happily munches dried skin cells and causes far less aggravation. [Editor's Note: Don't forget, however, that many persons are allergic to the excretions of mites that can also live on pillow and mattress dust for long periods of time. By the way, have you ever taken a close look at the standard electron microscope snap-shots of these weird creatures. They look like they just stepped out of a B-class science fiction or horror movie.] And not all body parasites creep and crawl -- you can find fungi in your hair and mold in your skin folds if you look closely enough.

Inside your digestive tract you can find, among others, the protozoan that causes Amoebic Dysentery, 20-meter beef tapeworms, and a hookworm that has a penchant for finding its way into your bloodstream.

Other creatures in your blood can include the hermaphroditic Shistosoma worm, which can lead to a bloody and scarred bladder, while in your lymphatic system you may find the 12 cm Wucheria worm. In your liver you may come across the bile-loving Clonorchis sinensis fluke and, perhaps most horrifying of all, the brain can house Naegleria fowleri, an Amoeba that just loves the warmth that it finds inside your skull, reproducing in the millions until you simply drop dead.

In addition to his statistics, Rosebury treats us to a fascinating tour through little-known by-ways of literature and anthropology, documenting historical and cultural attitudes to obscenity -- regaling us with tales from Rabelais and other great scatologists.


1. Theodor Rosebury, Life on Man (Secker and Warburg, London, 1969).
2. Question by Roger Taylor, "Bodily Breeding: How Many Different Species Live on or in the Average Human Body and What is the Total Population of These Guests?," New Scientist (September 30, 1999)[].
3. The New York Times, Science Section D (October 2, 2000).
4. The Los Angeles Times, p. B2 (October 26, 2000).
5. Paul Taylor (January 28, 2000) [ ].
6. Roger M. Knutson, Fearsome Fauna: A Field Guide to the Creatures That Live in You (W. H. Freeman; 1999)
7. Roger M. Knutson, Furtive Fauna: A Field Guide to the Creatures Who Live on You (Penguin; 1992)(Ten Speed Press; 1996).