According to the manipulation hypothesis, a number of parasites purposefully and specifically alter the behaviour of their hosts and thus increase the probability of their transmission to an uninfected host. For example, it is assumed that toxoplasmosis can reduce fear of cats in infected rodents, or reduce the speed with which they can react to simple stimuli. Parasites transmitted by sexual intercourse could increase the sexual activity of their hosts or the attractiveness of infected males for females. In some cases, parasites affect the behaviour of their hosts directly, e.g. by targeted interventions into the nervous system (rabies), in some cases indirectly; for example, the bacteria causing the plague damage the oral system of fleas so that they can bite, but cannot suck blood, an infected flea is therefore constantly hungry and bites and thus infects more hosts.