Epidemics occur when an infectious
disease spreads beyond a local population, lasting longer and reaching
people in a wider geographical area. When that disease reaches worldwide
proportions, it's considered a pandemic.
Several factors determine whether an outbreak
will explode into an epidemic or pandemic: the ease with which a microbe
moves from person to person, and the behavior of individuals and societies.
On the global level, different populations interact through travel, trade, and war—all opportunities for microbes to reach new areas. Rapidly growing cities also allow microbes to infect large groups of people. Of course, in many situations, individual and communal behavior also contribute to the spread of disease.
