Design the illness. (It's only a game.)
make it up: every setting below is yours
how many people one sick person passes it to, in a town carrying on as normal. Set it here and How catching follows; change the catching distance, how catching it is or how long the illness lasts and it follows them
how close someone has to be to a sick person (in the same building, or in sight outdoors) to risk catching it
the chance of catching it for each minute spent that close
how long after catching it someone falls sick and starts spreading it (they show nothing till then); each case is a little different, up to a fifth sooner or later
how long the sick are ill, weakening as it goes, before they die or recover; each case is a little different, up to a fifth shorter or longer
the share of the sick who die of it; the rest recover and are immune
how plain it is that someone's sick. Low: the town hardly notices till people start dying, and panics slowly. High: everyone can see it, and the town takes fright fast -- keeping away, staying home and boarding up far sooner