That is in fact one of my memories of Greece. It was fortunately just before leaving. Playing back the prior day, I think I figured out the cause. On way back from Delphi to Athens the bus stopped at a restaurant. Now it could have been in the food, but I found it usual that the waiter unscrewed the cap off the bottled water that he served me. They were too busy to allow for such niceties. I suspect they were too cheap to use real bottled water and refilled bottles with less than safe (for visitors) tap water.

When visiting Mexico, we call that Montezuma's revenge.

And yes, it's 2% tail of EU trying to shake the rest of the 98% dog.