Because you end up losing the better part of half a week to it...
I started my GP placment last week. We are attached to two practices - an 'inner' practice, which is city centre based (Birmingham, Dudley, Wolves) and has a group of final years, where we consult and have tutorials, and an 'outer' practice which is supposed more rural, but mine is basically still Birmingham, in a 'nice' area. I'm the only student there, and all my time is spent consulting.
I started at my outer practice last Monday, and instantly felt at home, becasue the practice is so warm and welcoming. I had a good first three days, and then should have gone to my inner practice on Thursday and Friday. Unfortunately, on Wednesday evening, after my choir rehearsal, I started to feel extremely nauseous. By 10.30pm, I had my head down the toilet puking my guts up, and returned there at regular intervals throughout the night. And I mean regular - every 20 minutes for the first four hours. It was the worse night of my life. Thank God for ensuites!
Anyway, to cut a long story short, I spent Thursday day sleeping, Thursday night sleeping, and Friday morning sleeping. After that I felt much better, though a little wobbly. It wasn't until yesterday, when I was bouncing around with my usual vim and vigour, that my housemate remarked I 'was back to normal'.
Of course the moral of the story is: Don't bother your GP with diarrhoea and vomiting. You'll only end up sharing the joy.