It has been shown that virulence factors can be transferred between microbes. The evidence for this is derived from the occurrence of pathogenecity islands in various bacterial species. These pathogenecity islands are blocks of genes within the chromosome that code for virulence. These genes code for a variety of virulence traits, such as the ability to acquire iron and to adhere to or enter host cells. These segments of DNA appear to have been acquired in a single step from a foreign source.
Many host factors play a role in the emergence of an infection.
The herd immunity in the community affects the ability of the pathogen to spread and establish itself in the community. In the sixteenth century, up to a third of the indigenous population of the West Indies and Mexico died following contact with Europeans. The lack of herd immunity probably contributed to the high fatality rate among those who were infected with the Nipah virus. Human activity both social and economic is believed to play an important role in the emergence of infections. Changing eating habits can also have a great impact on the emergence of food-borne infections.