When I visited the US in 2008, on my first day I picked up Salmonella poisoning that stuck with me for the majority of the trip.
At the end of day 1 on this trip we decided to grab a quick dinner from Taco Bell. Not wanting to drink anything fizzy I plumped for the water which came from the same dispenser as fed the sodas. The water came out a little bit brown but I wrongly assumed it was it slightly mixed with one of the sodas and drank it.
This gave me a bad stomach for the majority of the first leg of the trip and without going into too much detail I probably rode more as many toilet seats as I did coasters for the next few days :)