influenza
"I got violently ill with Influenza A on Monday night.  Scary high temperature.  Went to the Urgent care Tuesday morning with a more moderate 101.4 temp.  Didn't feel better until Wednesday noon (3 doses) but today on Thursday I feel like I never had the flu.  Even my cough is better.  Side effects were kind of nasty though as I had extreme diarrhea that was as bad as what you get for colonoscopy prep.  So on Tuesday and Wednesday morning I would have told you it wasn't worth it.  But by early Wednesday afternoon I was glad I did it."