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, those that are permanently resident and 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.

In his seminal work Life on Man, the 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 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 ranges from 100 billion to 100 trillion.

From this figure it can be estimated that the microbial density on a square inch of human bowel is around 50 billion organisms.

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 intestines, from mouth to anus, plus eyes, ears, and airways.

It is stimated that 50 million individual bacteria live on the average square inch of human skin, describing the surface of the body as akin to a “teeming human population during Christmas shopping.”

However, this figure can vary widely throughout the almost 20 square feet that make up the surface area of a human.

In the oily skin that is found on the side of the nose or in a sweaty armpit, the figure can increase tenfold; and 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 genitourinary surfaces, the populations of organisms are much thinner. No microbial life have been detected at all in the 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; and all those on the inside would fill a vessel with a capacity of a mere 10 ounces.

If disease organisms such as viruses or other infections are present, these figures increase, but not by any significant amount.

The total number of organisms living on us is huge, but when one considers the volume of the human body, the volume of species using us as home is not so 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 a professor of microbiology based at the Queen’s University of Belfast, reckons that the figure is in excess of 200.

“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 France, suggest that at least another 80 live in the gut, with many others living on our skin. Its impossible to be precise, but our permanent resident population certainly exceeds 200 species.”

“The human genome carries a maximum of 100,000 genes, yet the average bacterial genome has 2,000 genes. Therefore there are actually four times as many genes found in the bacteria that live on humans as there are in the human genome itself.”

Of course, it’s not just bacteria and viruses that make people their home.

In the books Fearsome Fauna and Furtive Fauna, the author describes the wide range of parasites that live both on and inside you. 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. 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 irritation. 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, among others, find the protozoan that causes amebic dysentery, 60 foot-long 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 five-inch 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 its millions until you drop down dead.