Heavily traveled international borders, like the one seen here between Mexicali, Mexico, (right) and Calexico, California, (left) can present acute public health concerns. Migrants from Mexico, where health care is often poor and vaccination rates low, can carry diseases that have been nearly eradicated in the United States. Likewise, U.S. travelers can introduce new diseases to rural Mexico, where they can spread quickly. The smallpox virus changed much of Mexican history when, in the early 1500s, Spanish conquistadors brought the disease to the New World. A subsequent epidemic wiped out much of the local population, bringing on the collapse of the Aztec Empire.