The hypothesis of Panspermia is that the Universe is full of the seeds of life, such as hardy bacteria, and that life forms on any hospitable planet where these bacteria land. Under this scenario, the origins of life can be pushed off to some distant planet, so we don't need to worry that the early Earth may have been inhospitable in the past. If true, this idea would also mean that we are closely related to any aliens in space.
Some of the proponents of panspermia claim that a constant "seeding" of the Earth by these alien bacteria and/or viruses could be responsible for the emergence of new diseases on Earth, such as HIV or bird flu.
Panspermia does have some severe problems that have yet to be tested. Space is a very, very harsh environment, and the distance between planets is further than our minds can comprehend. Can bacteria survive the millions or billions of years it takes to travel between planets? Can bacteria survive the impacts necessary to loft a chunk of rock into space? Can bacteria survive the fiery re-entry of a rock into a planet's atmosphere? And where did life originally come from?
I will admit that I am a doubter of Panspermia. Note that I do not claim that Panspermia is wrong, but I will claim that I think it to be wrong. This is a hallmark of science. Ideas can be proven to be wrong, but no proof has yet emerged for or against Panspermia. Time will tell.