If you think planning an exotic trip means making reservations, finding your passport, and buying a guidebook, you're missing a vital step. Unless your destination is in North America, Europe, Australia, or New Zealand, a visit to a travel clinic should be on your predeparture list. There are serious, potentially fatal, infectious diseases that travelers can pick up, "even in destinations as close and well traveled as the Caribbean or Mexico," warns Susan Early, R.N. at George Washington University Travel Clinic in Washington, D.C. Take your best shot. "The most important…