Really nice idea! To echo what others are saying, I wish the gun play had more purpose, as switching hosts and never firing was the easiest way of progressing. In fact killing people is practically detrimental to you, as it robs you of a host and can leave you stranded.
Perhaps making shooting the means by which you switch hosts could help? Maybe the parasite shoots goo at people or something.. idk that's kinda weird, but something to think about if you ever continue!
Note: Had a bit of trouble running the game so I played the web version that was uploaded after.