Invited talk: etiquette for canceling
If you're seriously ill and risk either your health or the health of the audience, the correct course of action is to cancel. Just remember to add many apologies to the email/phone call.

It's up to the organizers of the conference to manage risks -- including cancellations, and if they did a poor job at risk management, they'll know better next time.

Edit: By 'many apologies' I mean 2-3. One at the beginning of the message/call, one at the end.