FAQ for
The Invasion (2007) More at IMDbPro »

Both Oliver and Wendy Lenk (Veronica Cartwright), Dr. Bennel's patient, were immune. These two characters had two things in common, and it was never really clarified as to which one (or both) were responsible for the immunity:

1) Both characters had been afflicted with Acute Disseminated Encephalomyelitis (ADEM), which is a rare autoimmune response that follows a viral infection. The persons own immune system presumably cross-reacts with the white matter of their brain and spinal cord, producing multiple inflammatory lesions. Having had this immune response could have been a sort of coincidental "vaccination" toward the alien disease.

2) Both characters were on the benzodiazepine, clonazepam, which is a potent anti-anxiety drug. (A major side effect is a "zombie-like" demeanor, which I thought allowed the film to make an interesting comment on today's society, considering the huge amount of people being prescribed these drugs.) What makes clonazepam a possible explanation for immunity? Well, clonazepam can also be used for night terrors, which was the reason Oliver was taking it, because, although it induces sleep and is often taken for insomnia as well, it paradoxically disturbs stage IV sleep, the deepest state of sleep, during which night terrors occur. This alien disease takes over its host during sleep, and I kept expecting them to discover that it was actually during stage IV sleep, as opposed to REM sleep like Dr. Galeano kept saying, which could have explained the clonazepam as a mechanism for Oliver and Wendy's immunity. Another thing that led me in this direction was a scene at the very beginning of the movie (a sort of flash-forward) of Dr. Bennell in the pharmacy finding a bottle of clonazepam and taking some. This is during the time when she's trying NOT to fall asleep, and, since clonazepam works as a sedative, I was assuming that Dr. Bennell had made the same connection I had and was hoping to prevent herself from entering stage IV sleep. But when this scene finally came up in the course of the film, it seemed like the clonazepam theory had been dropped.

So did Dr. Galeano discover a cure using ADEM anitbodies or did they pump everyone full of clonazepam? Perhaps an explanation would have been over the heads of a general audience, but there were just enough hints that a science geek like me had a good time piecing things together.