Setting: dystopian near-future London in December. About 50 or so survivors are living in a Tube station.
What I'm looking for is an easily foreseeable, easily treatable (if they had access to hospitals or antibiotics, which they don't) epidemic with potentially devastating consequences. Basically they've just had a narrow escape, think they're relatively safe, and then people start getting sick. It should be something pretty obvious—one of the characters, who has no medical training, spots it immediately.
I do want most—but not all—of the characters to survive, so it has to be something that isn't necessarily fatal but could easily be, particularly for a malnourished population.
What would the first signs be? What would people try to do about it (beyond, in this case, going up to the surface for help, which is the point of this whole plot point).
Thanks in advance!