"Montezuma's Revenge" is the most common health ailment experienced by foreign visitors to Mexico. It is a mild form of diarrhea, although it does come in a variety of strengths and can last from a day to a week. A change of environment (food, climate) can be the cause of this, and you should not be concerned if it does. More severe forms are acquired by drinking non-purified water (the most common way) or foods that have not been properly prepared. Provided that you eat sensibly (only at good restaurants, hotels), watch the water you drink, and don't overdo it on the spicy food, you should be fine. If you do become affected, the best thing is lots of liquids (not much food). Buy some hydration drinks with added salt, sugars and electrolytes that help your body to re-hydrate. This is important when you have diarrhea as your body is constantly releasing fluids. Available at all pharmacies, with a choice of artificial flavors.