Many experts say it is only a matter of time before there's another major influenza pandemic. From year to year, the virus changes only slightly, so most healthy people develop a certain level of resistance to the annual flu bug. They may get really sick but it won't kill them unless their overall health is already compromised. However, new deadly strains do emerge, particularly when the virus jumps back and forth between animals and people. In this cross-species exchange, the virus can pick up new genetic traits from flu viruses that normally attack birds, pigs or other animals. This gene-swapping can produce an especially virulent strain. In the past century, three major flu pandemics struck the human population.
Some experts believe the so-called H5N1 bird flu strain now circulating in Asia will be the origin of the next pandemic. Scientists have predicted a death toll ranging from fewer than 2 million to 360 million.
Hong Kong flu, 1968-1969
The Hong Kong flu virus resulted from bird and human flu viruses exchanging genes. It was first detected in early 1968 and claimed between 750,000 and 1 million lives. It was the mildest of the three 20th-century flu pandemics. In some ways, the Hong Kong flu was similar to the Asian flu, which might have provided people with some immunity to it.
Began in mid-July, 1968, in southeastern China
Spread to Hong Kong in the same month, reached maximum intensity within two weeks.
Spread worldwide, but clinical symptoms mild and mortality low; disease spread slowly.
The United States epidemic began in California in September, carried by troops returning from Vietnam. In Europe, the epidemic began in December in Britain, and progressed until early April, 1969.
Total estimated death toll: 1 million
Asian flu, 1957-1958
The Asian flu originated in China in 1957 and spread worldwide that same year. The virus lasted until 1958. It is believed to have passed through pigs and birds before infecting humans.
Broke out at the end of February, 1957, in a single province in China.
Spread throughout China in March, and reached Hong Kong by the middle of April.
Within six months, every part of the world had experienced cases.
European and U.S. epidemics exploded with the opening of schools in September, but the first wave ended by December. The second wave appeared between one and three months after the first, and affected mostly the elderly.
Total estimated death toll: 2 million
Spanish flu, 1918-1919
The Spanish flu pandemic was an unusually severe and deadly strain of influenza that killed between 25 million to 50 million people worldwide in 1918 and 1919, though the toll may have climbed as high as 100 million. It is thought to have been one of the most deadly pandemics in human history. The latest research suggests an avian virus acquired mutations that allowed it to infect humans.
First wave: simultaneous outbreaks in March, 1918, in Europe and in different parts of the United States.
Infection travelled back and forth between Europe and the United States via troop ships, then spread to Asia and Africa.
Highly contagious, but not especially deadly.
Second wave: near the end of August, 1918, simultaneous outbreaks in France, Sierra Leone and the United States with a 10-fold increase in the death rate. Most deaths in young and healthy persons between ages of 15 and 35.
Spread throughout the world, except to Australia, which maintained a strict maritime quarantine, so the flu arrived there later, at the start of 1919, and was milder. But it lasted longer than elsewhere.
25 per cent of the world's population fell ill.
Total estimated death toll: 25 million to 100 million
The World Health Organization, in assessing the epidemic threat in January, 2005, estimates
2 million to 7.4 million deaths
David Nabarro, United Nations co-ordinator for avian and human influenza, in a news conference, Sept. 29, 2005, estimates 5 million to 150 million deaths.
Michael Osterholm, director of the Centre for Infectious Disease Research and Policy at the University of Minnesota, estimates between 180 million and 360 million deaths.
SOURCES: THE WORLD HEALTH
ORGANIZATION, UNITED NATIONS AND THE
CENTRE FOR INFECTIOUS DISEASE RESEARCH