Reminds me of another film.

K-Pax
http://www.imdb.com/title/tt0272152/?ref_=fn_al_tt_1

Not an invasion film, but, it has similarities.

You're never sure if there's truly an alien inside of this person, or if they are sick because of a major trauma earlier in their life.

The film is great though if you like The Host, I highly recommend it.