Robyn Bailey, age 15, of Santa Cruz, Calif., for her question:

WHAT EXACTLY IS AN EPIDEMIC?

An epidemic is the outbreak of a disease that attacks many people at about the same time. It may spread through one or more communities.

When the disease spreads through an entire region, it is endemic. If it spreads over the world, it is pandemic.

To fight epidemics, medical people often immunize residents against the disease. They also quickly treat those with the disease and enforce quarantines to stop germs from spreading.

An epidemic of bubonic plague caused more than 60 million deaths during the Middle Ages. But as the people discovered that the disease was being caused by fleas from infected rats, the rats were killed, and it wasn't long until the epidemic was over.

Margaret Bell, age 14, of Prescott, Ariz., for her question: