West Nile virus
The West Nile virus stays alive in nature by getting spread back and forth between birds and mosquitoes.  Birds get WNV more often than any other living things.  When a mosquito bites a bird that has WNV, the mosquito gets the virus.  Then, if the mosquito bites a bird that does not have WNV, the mosquito can give that bird the virus.  In this way, birds and mosquitoes give the virus to each other and keep it alive.