My "flu" turned out to be a norovirus, the type of food-borne virus associated with cruise ships. I didn't go on a cruise, but it's fair to guess I picked it up on the previous week's travel to and from San Francisco.
I'm still considered contagious until Wednesday, according to the doctor I saw this morning. (And I still feel pretty awful.) The doctor wrote a letter to American Airlines and I cancelled tomorrow's flight to Florida. Even with a doctor's letter, cancellation still involves penalties, complications, and requires that you re-book on the same airline; however, I had $15 trip insurance, which bypassed the airline bureaucracy, netting me a full refund.
Back to bed...thanks to all of you who've emailed with "get well" messages.