Carrie Refiner, age 9, of Brooklyn, N.Y., 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 spreds over the world, it is pandemic.

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

An epidemic of bubonic plague caused more than 60 million deaths during the Middle Ages. But as the people discovered the disease was being caused by fleas from infected rats, the rats were killed and the epidemic stopped.