Wow, my 3 hour check-in that I posted turned into a lot longer. Just read everything up till now, and I have to agree with Thucy about the disease idea. I really just couldn't buy into it myself. Even without direct exposure to diseases, many anti-bodies and defenses are inherited from the parents; or at least the mother.
I like the idea of a zealot trying to hijack the time machine, but I'm thinking he'd become more of an unwilling, and perhaps, unknowing pawn of someone else. Maybe this was the secret goal of one of the scientists when they found our zealot character.
Or, pardon the Doctor Who reference, but maybe one of the scientists want to "kill Hitler" in a sense, and considered this zealot the perfect means to that end. Of course, that may have been his plan, but things go horribly wrong when our zealot turns out to champion some other cause, or goes so completely psycho, that our scientist can't control him.
At this point, zealot person takes control of the time machine and leads whoever is there at the time, into another time. Maybe he misses his target time or place, and they collect a 3rd Party member. Being inexperienced, the zealot kidnaps said person who has seen the machine, and drags them along too. It would all depend on our exact method of time travel if this idea would even be viable. But I think it's an enjoyable twist on Thucy's past person taking control.