One of the biggest things keeping humans from any kind of real space travel is the restrictive nature of our bodies - we're just too much of fragile, bags of meat to make that kind of trip. For viral aliens, that's not a problem. The essential character of these critters is the ability to transmit their intelligence through a single cell of their body. The best example of this is, of course, John Carpenter's classic The Thing, where the creature can mimic the forms of what it eats and needs to be burned to be killed. The Black Oil virus from The X-Files is also an alien life form used as a transitional stage for the Colonists.