Should I notify students that I've interacted with after getting sick, by email? I saw a bunch of students today during my office hours (~15-20 out of a class of 96). Then around noon I went downhill quick, and now I'm sporting a fever over 100 F.

I don't know necessarily what I have, but it seems reasonable to assume it was already there when I had close interaction with the students this morning.

Would you send an email to students notifying them? Would you wait until you went to the doctor (which won't happen until tomorrow, if I go)? If you would email them, how/what would you say. Note, I also don't know exactly who all I saw, so I was going to send a class email.
Either you infected them, or you didn't. Sending an e-mail about it will not change that (enough time has passed that telling them to wash their hands now will make no difference).

Therefore, it only makes sense to send an e-mail if it turns out to be something where early treatment can make a big difference, or where ignoring it can do harm. (IIRC, this year's flu is an especially bad one, and students might be tempted to dismiss it as just a cold. It's good to remind people that the flu kills and that they need someone to check in on them every few hours.)

Technically, an early warning might also let them avoid people until they are sure they aren't infectious, but in practice that is unlikely to happen as "I was in the same room as someone with fever earlier" probably won't be accepted as a valid reason to skip class...