Always choose your meals and snacks wisely. Most likely your body will need to go through a period of adjustment as it's exposed to new bacteria. Street foods, such as tacos, are both tempting and tricky. Hygienic conditions are sometimes very poor in sidewalk food stands. Be selective when choosing to eat food on the street. Choose places that are recommended by other people (where you see many local people). Be especially cautious with water-based drinks, and fruit drinks prepared with water as its base. Only eat fresh fruits or vegetables if cooked or peeled.
Some people come to Mexico and eat street food, never suffering much more than an occasional upset stomach (if at all); it depends on your body's digestive system, immunity, and how courageous you are with your eating habits! Be careful but don´t miss out on the delicious Mexican food!