Many bacteria and higher eukaryotes live in harmony in a symbiotic relationship that benefits one or both of the partners. Indeed, we are colonised by bacterial cells which outnumber our own cells ten to one. This amicable bacterial lifestyle contrasts with a pathogenic one, in which the bacterium causes damage to its host. This is a potentially dangerous strategy for a bacterium, because the provoked host is capable of fighting back. A pathogenic lifestyle offers short-term gain. By outcompeting other bacteria within its host, the bacterium can achieve local dominance and, by more widespread colonisation, expand its territory more globally. However, evolution has to balance these advantages against the possibility that the bacterium is eliminated. The latter could occur if the bacterium is too weak to prevent its destruction by the strong host defences it has incited or if its potent virulence wipes out the host and, thus, its source of food.

As pathogenicity appears to be such a risky business, it may be an abnormal condition. Evidence to support this view comes from several lines. First, many of the genes involved in virulence appear to be relatively new – that is, new to the organism made pathogenic by their presence. These genes are frequently found on mobile genetic elements such as plasmids, phage, and pathogenicity islands that have recently been acquired by the organism.