in the host, causing much harm (disease and death). If the pathogen's virulence kills the host and interferes with its own transmission to a new host, virulence will be selected against. But as long as transmission continues despite the virulence, virulent pathogens will have the advantage. So, for example, virulence often increases within families, where transmission from one host to the next is likely, no matter how sick the host. Similarly, in crowded conditions such as refugee camps, virulence tends to increase over time since new hosts cannot escape the likelihood of infection. Coincidental evolution hypothesis Some forms of pathogenic virulence do not co-evolve with the host. For example, tetanus is caused by the soil bacterium Clostridium tetani. After C. tetani bacteria enter a human wound, the bacteria may grow and divide rapidly, even though the human body is not their normal habitat. While dividing, C. tetani produce a neurotoxin that is lethal to humans. But it is selection in the bacterium's normal life cycle in the soil that leads it to produce this toxin, not any evolution with a human host. The bacterium finds itself inside a human instead of in the soil by mere happenstance. We can say that the neurotoxin is not directed at the human host. More generally, the virulence of many pathogens in humans may not be a target of selection itself, but rather an accidental by-product of selection that operates on other traits, as is the case with antagonistic pleiotropy. Expansion into new environments A potential for virulence exists whenever a pathogen invades a new environment, host or tissue. The new host is likely to be poorly adapted to the intruder, either because it has not built up an immunological defense or because of a fortuitous vulnerability. In times of change, natural selection favors mutations that exploit the new host more effectively than the founder strain, providing an opportunity for virulence to erupt. Host susceptibility Host susceptibility contributes to virulence. Once transmission occurs, the pathogen must establish an infection to continue. The more competent the host immune system, the less chance there is for the parasite to survive. It may require multiple transmission events to find a suitably vulnerable host. During this time, the invader is dependent upon the survival of its current host. The optimum conditions for high virulence would be a community with immune dysfunction (and/or poor hygiene and sanitation) that was