I read them in junior high (like, late 90s) so I *think* they're written for teens...
The first book in the series involves a fever of unknown origin. It's making people especially sick and runs very high. In the end we find out that one of the businessmen brought it home after a recent trip out of the country. It's sad because his daughter was the first death.
The other ones involve a snowstorm and an earthquake...and I think either a fire or flood. The titles are very simple. I keep thinking the first one is "fever" but I can't find it.
I'd love to read them again but I can't FIND THEM and it's driving me batty.