Water
Tap water is generally NOT safe to drink. Drink bottled or filtered. Brush your teeth with bottled water in the first few days. Most hotels provide filtered water (garrafón).
Food
Street food is generally safe at busy stalls with high turnover. Avoid raw vegetables washed in tap water and unrefrigerated dairy.
Mosquitoes
Dengue is the main mosquito-borne risk on the coasts and Yucatán. Use DEET repellent and cover up at dusk.
Altitude
Mexico City is at 2,240m. You may feel breathless and tired on day 1. Hydrate, take it easy and skip alcohol on arrival.
Sun
Mexican sun is intense year-round, especially in the Yucatán and Baja. SPF 50+ and a hat are essential.