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Why are viruses virulent? Viruses require cells to replicate themselves. They are parasites. They are also pathogens, entities that cause disease. Pathogens vary in their virulence, the extent to which they are harmful to their hosts. Pathogens may evolve reduced virulence if their probability of transfer to a new host at any given point in time is low. If a virulent virus had a low probability of reaching a new host, individual viruses that did not seriously harm their host would tend to persist. Highly infectious viruses such as those causing influenza will not tend to evolve reduced virulence.