ABSTRACT
Certain infectious diseases spread over continents, whereas others are found only in remote foci areas. For example, the inuenza virus can infect people anywhere in the world; vector-borne diseases like malaria, dengue, and yellow fever can be carried from the tropics to temperate zones by infected travelers, who act as reservoirs. In contrast with these widespread diseases, poliomyelitis and Guinea worm are now found only in a few areas of Africa and Asia, because of long-running global campaigns to eradicate them.
