it takes place in England which normally gets a good amount of rain.
so when it rains, the rainwater collects any infected blood, remains, etc. which then evaporates and turns into rain that rains onto normal people
also, infected can die in a river or something, and that can contaminate water supplies. even someone getting infected blood on them, then taking a shower can potentially put the virus in the water supply
then there's no knowing whether animals can get infected. for all we know, some infected person got bit by a mosquito, then that mosquito bit a normal person and that infected them somehow.