Enough to fill a big soup can. "That's three to five pounds of bacteria," says Lita Proctor, the program coordinator of the National Institutes of Health's Human Microbiome Project, which studies the communities of bacteria living on and in us. The bacteria cells in our body outnumber human cells 10 to 1, she says, but because they are much smaller than human cells, they account for only about 1 to 2 percent of our body mass—though they do make up about half of our body's waste.