
Some scientists think that an actual virus may be alien to our planet. It originally came from space or some other planet. Taking travel by means of Comet. It may sound crazy but reading the full theory makes a lot of sense. When I dug around for more information I found a lot of possible proof to back up these claims.
They say the tail of a comet drags space dust and debris possibly taking along a virus. One of the proofs that may back up this theory is the bird. Birds are usually the first species to contract a virus. Why?, When a comet passes along our planet some of that trail it drags along makes it into our atmosphere, taking around a year or two to fully reach our land level. However birds are at constant flight, passing through that alien virus while it lingers in our atmosphere.
Somewhere around the early 1900s , an influenza outbreak suddenly infected Europe, US, and parts of Asia simultaneously. The outbreak didn't seem to infect people like a one by one network, it infected people all at once.
Whatever the case, if this theory is right, than I guess we've been battling alien invasions for along time now.