Why diseases spread.

Every year, cases of the cold and flu seem to spread widely across the U.S. During the 1940s and 1950s, for example, polio was a national epidemic. Just how do diseases spread so rapidly?

According to Stan Silberg, an epidemiologist at the University of Oklahoma Health Sciences Center, methods include infectious droplets entering and leaving through the respiratory tract; agents of disease contaminating water, food, or milk and entering and leaving through the digestive system; sexual contact; and arthropod vectors such as mosquitoes, ticks, etc. The speed of the spread of infectious agents depends on the presence of a densely populated community, a highly susceptible population due to lack of immunizations, and insufficient medical care.

The rapid spread of diseases partially can be explained by modern transportation systems and the fact that people are traveling more than ever before. "Airplanes, especially, allow us to go from one end of the world to the other in a matter of hours. You can be incubating an organism--influenza, for instance--and, if you're on a crowded airplane, the people around you can get infected. Look at how many people are going to get off in various spots across the country. When they get off the plane, each individual has the potential of spreading the infecting agent to many others.

"Some of the most common communicable diseases--such as influenza, measles, tuberculosis, and common colds, as well as many others--are rapidly spread through the respiratory tract. Sexually transmitted diseases also are rapidly spread.... In addition to persons who are clinically ill from infections, there are those who are...