There is an evolutionary trade off between virus virulence and virus transmission. A virulent virus does a lot of damage to its host, and produces a lot of offspring. However, if the host's illness prevents the host from coming into contact that new hosts that the virus could jump to, the virus actually has relatively low evolutionary fitness. In contrast, a virus that is less virulent could infect far more hosts because the hosts are well enough to come in contact with many other potential hosts.
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