How do bacteria cause infection?

In order to cause disease, the pathogen or a disease-causing microbe entering the body must reproduce in sufficient numbers before it is overwhelmed by the body's immune response. Bacteria replicate using their own resources through a process called binary fission, in which one bacterium divides into two. Bacteria are noted for their adaptability; once a bacterium arrives in a suitable environment, it can divide and multiply very fast. Escherichia coli, a common intestinal bacterium, can divide every 20 minutes. Unlike viruses, most, but not all, bacteria operate outside cells. One example of a bacterium that can enter cells is the organism that causes leprosy.