Emerging diseases are those which have shown an increased in humans over the last 20 years. Re-emerging diseases are those which have reappeared after a period of significant decrease in incidence. The etiological agents of these diseases in the Western Hemisphere are viruses (HIV, dengue, oroupuche, sabia, guanarito, or hanta), bacteria (Vibrio cholera, Borrellia burgdorferi, Legionella pneumofila, Eseherichia coli 0157:H7, or other bacteria with a new pattern of antibiotic resistance), or parasites (Cryptosporidia, Cyclosporidia or drug resistant Plasmodium falciparum). Due to the widespread geographical distribution of these infectious diseases in the Americas, and an increasing number of travellers (more than 87 million persons within the region in 1997), there are many opportunities to contract an infection when travelling in developed or undeveloped countries. The infection may present with symptoms during the trip, or following the traveler s return to his or her place of origin. However, too often practicing physicians do not inquire about the travel history of their patients and, when they do, they often lack the information about diseases relevant to travelers. From the regional perspective, the emerging or reemerging agents that pose a higher risk to tourists or travelers are: 1) those that cause enteric infections; 2) sexually transmitted diseases; and 3) vector-borne diseases, including those present in ecotourism areas. Emerging and re-emerging diseases that physicians may encounter in their clinical practice while caring for travelers returning from different countries of the Western Hemisphere are briefly described (Lyme disease, legionellosis, dengue, yellow fever, P. falciparum malaria, cyclosporidiosis and cryptosporidiosis). This report attempts to draw attention to the fact that new clinical and etiological entities are present in several geographical areas of the Americas; to place each of these entities into an epidemiological context; and to end the misconception that only travel to poor countries carries a risk of acquiring an infection. By knowing which infectious agents occur in each area and the incubation period of each disease, the treating physician can often treat patients successfully. Health care professionals must be aware of the organisms circulating in the region so that they have them in mind during their clinical practice.