Only about 10 percent of the cells in the human body are actually human cells — the rest are fungi, bacteria, viruses or other non-human cells. The human gut alone has over 100 trillion microbial cells, but only about 1 percent of all the microbiota on and in the body has actually been identified. Researchers estimate that the non-human microorganisms in and on a human body outnumber the human cells by a factor of 10.
More facts about non-human cells:
Although more than 3 million types of microbes are thought to exist, fewer than 8,000 have been described. These microbes make up a full one-third of the Earth’s biomass.
People have unique bacteria signatures. Studies show that people tend to carry about 3,000 bacteria from about 150 species on their hands, but each of a person’s hands has different types of bacteria, with an overlap of less than about 15 percent.
More and more studies have shown that bacteria that were thought to exist only outside the human body actually do exist in the body. For instance, hydrothermal vent eubacteria, a type of bacteria that was once found only near deep sea heat vents, have been found on prosthetic hip joints.