You're assuming that viruses don't have natural reservoirs. For example, viral hemorrhagic fevers, particularly filovirus VHFs, are both highly transmissible an

You're assuming that viruses don't have natural reservoirs. For example, viral hemorrhagic fevers, particularly filovirus VHFs, are both highly transmissible and highly deadly -- accounting for their history of rapid, fast-growing, and quickly-diminishing epidemics amongst humans -- but have stable zoonotic reservoirs that allow them to circulate freely across those populations. (Bats' unique immune system, like camelids, makes them a particularly good evolutionary proving ground for viral evolution, so they serve as a reservoir and a source of ongoing viral mutations.)