Yeah, good point; the humans in Alien Resurrection said they can get all sorts of cures from the Alien's biology so maybe these creatures are resistant to all forms of disease. Though, there was a fungus in Aliens Labyrinth that could make these creatures sick, but there was a cure as well (it just didn't work very well). The Aliens in the EU are also telepathic and could even communicate a little to humans through dreams. A chestburster could even talk to its host as shown in Aliens Steel Egg.