FACTBOX: Some facts about past flu pandemics

(Reuters) - Health officials around the globe are closely watching an outbreak of a new kind of flu that has killed at least 60 people in Mexico and infected seven in the United States.

Health officials have been warning that a new strain of influenza that can pass easily from person to person could spark a pandemic, a global epidemic that could kills tens of millions of people. Experts agree another flu pandemic is overdue.

Here are some facts about past flu pandemics and pandemic threats:

* The 1918 Spanish influenza pandemic is the benchmark by which all modern pandemics are measured. Some 20 to 40 percent of the worldwide population became ill and more than 50 million people died. Between September 1918 and April 1919, it killed more than 600,000 people in the United States alone. In a normal flu season, about 36,000 people die in the United States, and 250,000 to 500,000 globally.

* While the very young and the very old are most at risk with seasonal flu, the 1918 pandemic primarily struck young adults. It disrupted the global economy. Many small businesses, which were unable to unable to operate during the pandemic, went bankrupt.

* The virus that caused the 1957 Asian flu pandemic was quickly identified, and vaccines were available by August 1957. The elderly had the highest rates of death. The Asian flu killed 2 million people globally, according to the World Health Organization.

* The 1968 influenza pandemic was first detected in Hong Kong. Those over the age of 65 were most likely to die. It killed an estimated 1 million people globally, according to WHO, making it making the mildest pandemic in the 20th century.

* In 1976, a strain of swine flu started infecting people in Fort Dix, New Jersey, and worried U.S. health officials because the virus was thought to be related to the 1918 Spanish flu virus. Forty million people were vaccinated but the program was halted after several cases of Guillain-Barre syndrome, a severe and sometimes fatal condition linked to some vaccines, were reported. The virus never moved outside the Fort Dix area.

* H5N1 avian flu is the most recent pandemic threat. It first surfaced in 1997 and continues to infect humans who have direct contact with chickens. The H5N1 or avian influenza virus does not spread easily from one person to another.

* Since 2003, H5N1 virus has infected 421 people in 15 countries and killed 257. It has killed or forced the culling of more than 300 million birds in 61 countries in Asia, the Middle East, Africa and Europe.

* WHO has six pandemic stages. A full-blown pandemic requires sustained, human-to-human spread over many countries of a new and serious virus.

-- Sources: U.S. Department of Health and Human Services; U.S. Centers for Disease Control and Prevention; World Health Organization

Editing by Maggie Fox and Eric Walsh