The occurrence of diseases that kill thousands or even millions of people dates back to 430 BC, when a wave of typhoid fever wiped out a quarter of the population of Athens and weakened its world dominance. In the 1300s, the Black Death killed 20 million Europeans in six years.

In more recent history, despite the evolution of modern medicine, pandemics have persisted. Here are a few of them:

1852-1860: Cholera - This pandemic in Russia was responsible for more than 1 million deaths.

1918-1919: Spanish Flu - Some 20 percent to 40 percent of the world's population became ill. More than 20 million died, including 500,000 in the United States.

1957-1958: Asian Flu - First identified in China, the Asian flu spread quickly to the United States, where it killed 69,800 people, particularly the elderly.

1968-1969: Hong Kong Flu - Originating in Hong Kong, this flu caused about 34,000 deaths in the United States. Its viruses, identified as Influenza A,still circulate today.

1981-present: HIV/AIDS - Most extensive in Africa, this disease is also affecting smaller populations in other countries worldwide. Some 22 million are estimated to have died so far, including 500,000 in the United States.

1997 - present: Avian Flu - Originating in chickens in Hong Kong, this flu so far has killed more than 100 people.

2003:SARS Scare - Worldwide, concerns surfaced that a new, highly contagious form of pneumonia called SARS might become a pandemic. While it never reached that proportion, it did spread across the globe,infecting more than 8,000 people, about 80 of whom died.

SOURCES: U.S.Dept.of Health & Human Services; wikipedia.org