As hosts are immunologically naïve to pathogens they have not encountered before, emergent viruses are often extremely virulent in terms of their capacity to cause disease. Their high virulence is also due to a lack of adaptation to the new host; viruses normally exert strong selection pressure on the immune systems of their natural hosts, which in turn exerts a strong selection pressure on viruses. This coevolution means that the natural host is able to manage infection. However, when the virus jumps to a new host (e.g. humans), the new host is unable to deal with infection due to a lack of coevolution, which results in mismatch between host and virus immunomodulators.