A maddening disease.

When a cow in Washington State was diagnosed with mad cow disease last December, the news got a lot of attention.

Some Americans stopped eating hamburgers and steaks, and beef sales across the U.S. decreased. About 30 countries banned the import of American beef.

U.S. health officials say there is no need to worry. Officials believe that only one U.S. cow was infected. All beef products from that cow were recalled.

Mad cow disease is an illness that destroys a cow's brain cells. The animal then loses weight, acts strangely, and sometimes becomes paralyzed, or incapable of moving. Eventually, the cow dies. If humans eat beef from an infected animal, there is a small possibility that they could become sick, too.

Mad cow disease was first seen in Great Britain in 1986. Since its outbreak, more than 150,000 cows in Europe have been infected with the disease. The infected cow in Washington was the first known case in the U.S. An investigation revealed that the infected animal was born on a Canadian farm.

To protect the U.S. food supply, Congress has passed a law requiring all beef sold in U.S. retail stores to be labeled with its country of origin. The law takes effect this fall.