Eating fresh fruit in Mexico?

This may be a bit stupid, but I am concerned about tourist diarrhea while vacationing in Mexico. From what I've read it is safer if I buy whole fruit that has to be peeled. OK, but does the fruit have to be sanitized before peeling? If so, how? Also, can I consume fresh fruit juices or frozen fruit bars from anywhere? One more thing: how do people who stay in Mexico long-term cope with this? Do their bodies build up immunity? Sorry for all the questions. Thanks.