According to the U.S. Department of Health & Human Services,
Pandemic flu is virulent human flu that causes a global outbreak, or pandemic, of serious illness. Because there is little natural immunity, the disease can spread easily from person to person.

Pandemic flu is very different than its counterparts, Seasonal flu and Avian (or Bird) flu.

Seasonal (or common) flu is a respiratory illness that can be transmitted person to person. Most people have some immunity, and a vaccine is available.

Avian (or bird) flu is caused by influenza viruses that occur naturally among wild birds. Mild viruses are common in birds and cause few problems. More serious viruses can be deadly to domestic fowl and are sometimes transmitted from birds to humans. There is no human immunity, and no vaccine is available.

Currently, there is no pandemic flu, but the United States and the world experienced pandemic flu three times in the 20th century.

The 1918-1919 pandemic was the most severe; it caused at least 675,000 U.S. deaths and up to 50 million deaths worldwide.

The 1957-1958 pandemic was moderately severe, causing at least 70,000 U.S. deaths and 1-2 million deaths worldwide.

The 1968-1969 pandemic was the least severe; it caused at least 34,000 U.S. deaths and 700,000 deaths worldwide.

Health experts predict that pandemic influenza will attack us again and that, this time, our increasingly mobile population could be affected in a ways never seen before.