Bird flu spreads.

The disease known as "bird flu" is continuing to spread around the world. In February, scientists reported an outbreak of bird flu--also called avian influenza--in Africa.

Scientists think the virus that causes bird flu may have hitched a ride to Nigeria and neighboring African countries with wild birds that migrate there each year from Asia and Eastern Europe, regions where bird flu has occurred. Once in Africa, infected birds spread the disease to other birds because they shed large amounts of the virus in their waste and saliva. Animals that come into contact with these secretions are at risk of contracting bird flu, which can be deadly to domestic birds and even humans.

Scientists had been especially concerned that the virus would make its way to Africa. There, people keep millions of chickens in open-air coops. These birds can mingle with migrating fowl and become infected.

Sick poultry are a double whammy for poverty-stricken Africans. The birds are a crucial food source. Plus, infected chickens increase the risk that humans who handle the birds will catch the disease, says Joseph Bresee, an influenza expert at the Centers for Disease Control and Prevention. About half of the people who have become infected have died.

Experts are now destroying infected poultry and keeping healthy birds away from migrating birds.