Russian cosmonauts are claiming they have found bacteria on the outside of the International Space Station that have to have come from space. At least, they didn’t get on the ISS until the ISS was in space, meaning they already had to be there. Reading between the lines, that doesn’t mean the bacteria didn’t make their way from Earth to space at some point in the past, before the ISS was launched.
I wonder if harmless bacteria could go into space, mutate into something dangerous, then make their way back to Earth on a returning spacecraft. Has that story ever been done?
But The Andromeda Strain had a lame ending, as I recall.