The human body is thought to contain ten times more bacteria than human cells!

The number of bacteria living inside the body of your average healthy adult human are estimated to outnumber human cells 10 to 1. The surface of your skin alone contains an estimated 500 varieties of bacteria! However, ten major species predominate and account for about half of the bacterial population. Varying levels of these microbes in individuals may play a role in digestive maladies, gum disease, and skin disorders. Scientists have even found a correlation between the ratio of certain types of bacteria with obesity!
(source)