Avoid unpurified water. The tap water in many areas of Mexico is contaminated with bacteria and pesticides. Drink bottled water whenever possible. If this isn't possible, drink soda, mineral water or fruit juices. Don't forget that ice is made of water. Many restaurants have ice made of purified water, but make sure this is the case before chilling that soda with it.
Be careful during hygiene. Avoiding unpurified water extends to your toiletry routines as well. Brush your teeth with bottled water and keep your mouth closed during showers. Also wash your hands often while making sure to use plenty of soap.
Choose your dining carefully. Avoid any meal with raw vegetables unless the menu says they purify their vegetables. Watch for warning signs of contamination such as flies and lack of convenient soap access. If possible, dine at a restaurant that has received the Distintivo "H" award from the Mexican Tourism Board. These restaurants have proven their ability to maintain quality and hygiene.
Stay hydrated and rested. This may mean limiting some pleasure activities such as sunbathing and alcoholic drinks-just remember to keep it in moderation. Don't overbook yourself and allow plenty of time for sleep and rest stops.
Be careful of wildlife. Bring plenty of mosquito repellent containing DEET and sleep only in screened rooms or under a mosquito net. Don't pet strays and avoid touching any ocean wildlife, including coral. If you're bit, scratched or licked by any warm blooded animal, wash your hands immediately. If you're bit by a jellyfish or sea lice, rinse the affected area with vinegar.
Be proactive with treatment. If you begin to feel unwell, seek a doctor's advice. Though it will most likely be food poisoning or dehydration, there are more serious illnesses, such as hepatitis and some water borne illnesses, that are best to treat sooner rather than later.
