factor is the presence of multiple infections in a single host leading to increased competition among pathogens. In this scenario, the host can survive only as long as it resists the most virulent strains. The advantage of a low virulence strategy becomes moot. Multiple infections can also result in gene swapping among pathogens, increasing the likelihood of lethal combinations. Evolutionary hypotheses There are three main hypotheses about why a pathogen evolves as it does. These three models help to explain the life history strategies of parasites, including reproduction, migration within the host, virulence, etc. The three hypotheses are the trade-off hypothesis, the short-sighted evolution hypothesis, and the coincidental evolution hypothesis. All of these offer ultimate explanations for virulence in pathogens. Trade-off hypothesis At one time, some biologists argued that pathogens would tend to evolve toward ever decreasing virulence because the death of the host (or even serious disability) is ultimately harmful to the pathogen living inside. For example, if the host dies, the pathogen population inside may die out entirely. Therefore, it was believed that less virulent pathogens that allowed the host to move around and interact with other hosts should have greater success reproducing and dispersing. But this is not necessarily the case. Pathogen strains that kill the host can increase in virulence as long as the pathogen can transmit itself to a new host, whether before or after the host dies. The evolution of virulence in pathogens is a balance between the costs and benefits of virulence to the pathogen. For example, studies of the malaria parasite using rodent and chicken models found that there was trade-off between transmission success and virulence as defined by host mortality. Short-sighted evolution hypothesis Short-sighted evolution suggests that the traits that increase reproduction rate and transmission to a new host will rise to high frequency within the pathogen population. These traits include the ability to reproduce sooner, reproduce faster, reproduce in higher numbers, live longer, survive against antibodies, or survive in parts of the body the pathogen does not normally infiltrate. These traits typically arise due to mutations, which occur more frequently in pathogen populations than in host populations, due to the pathogens' rapid generation time and immense numbers. After only a few generations, the mutations that enhance rapid reproduction or dispersal will increase in frequency. The same mutations that enhance the reproduction and dispersal of the pathogen also enhance its virulence