Zombies tend to work when it's "everyone's already infected you just need to die" à la the Romero films (or The Walking Dead), but when it's only passed on by physical contact I have to question if it would be able to spread across the entire planet (depending on the incubation time, I suppose).
Aliens need to physically get to you to kill you or expand their numbers. Seems like it would be relatively easy to contain once you knew where you stood, especially with the kind of futuristic weapons systems they are implied to have by the 22nd century.