Epidemic: An epidemic is a disease outbreak in which some or many people in a community or region become infected with the same disease. Some epidemics occur when an entirely new disease, such as AIDS, or a new version of an old disease, such as influenza, emerges.

Pandemic: A pandemic is an epidemic that spreads throughout the world, as influenza did in 1918. If the source of the pandemic is a new virulent pathogen or a new form of an old virulent pathogen, very few people, if any, may be resistant to the disease, and the rates of illness and death may be high around the world, unless effective prevention strategies can be rapidly developed and implemented.

How Often Do Pandemics Occur?

Based on historical patterns, influenza pandemics can be expected to occur, on average, three to four times each century when new virus subtypes emerge and are readily transmitted from person to person. However, the occurrence of influenza pandemics is unpredictable, according to the World Health Organization. Experts agree that another influenza pandemic is inevitable and possibly imminent.

Flu Pandemics During The 20th Century

During the 20th century, the emergence of new influenza A virus subtypes caused three pandemics, all of which spread around the world within 1 year of being detected.

1918-19, "Spanish flu," [A (H1N1)], caused the highest number of known influenza deaths: more than 500,000 people died in the United States, and up to 50 million people may have died worldwide. Many people died within the first few days after infection, and others died of complications later. Nearly half of those who died were young, healthy adults. Influenza A (H1N1) viruses still circulate today after being introduced again into the human population in the 1970s.
1957-58, "Asian flu," [A (H2N2)], caused about 70,000 deaths in the United States. First identified in China in late February 1957, the Asian flu spread to the United States by June 1957.
1968-69, " Hong Kong flu," [A (H3N2)], caused about 34,000 deaths in the United States. This virus was first detected in Hong Kong in early 1968 and spread to the United States later that year. Influenza A (H3N2) viruses still circulate today.

Both the 1957-58 and 1968-69 pandemics were caused by viruses containing a combination of genes from a human influenza virus and an avian influenza virus. The origin of the 1918-19 pandemic virus is not clear.

Additional Resources:

• HHS: Pandemic Influenza
• CDC Information About Influenza Pandemics
• WHO: Avian Influenza Fact Sheet

Sources: Centers for Disease Control and Prevention, the Department of Health and Human Services, and the World Health Organization