I think the aliens are some what immune. I think it makes them sick in a different way. Probably gives them a bit of a fever and wares them down but doesn't turn them into zombies ^^ sort of like a 24 hour flu. Maybe its the mix of alien and human DNA that makes the right genetics for complete immunity though. Of course the whole part alien makes me think of April from the TMNT show on Nick but I don't have a problem with it. I don't think Rex knows about it though.