Influenza pandemics are rare but recurrent events. In the last century there were three such pandemics in which millions of people died.
The 1918 Spanish flu is considered one of the deadliest disease events in human history, in which as many as 50 million people died.
These numbers dwarf fatalities from swine flu, but scientists caution that it is too early to predict how the virus may continue to evolve.
Each of the 20th Century pandemics lasted around a year, though the Spanish flu of 1918 saw second and third waves that stretched into 1920 causing further fatalities.
1918 SPANISH FLU
- First reported in America in March 1918
- Spread through rest of world by June 1918
- Infected 50% of the world's population
- Killed 40-50m people
1957 ASIAN FLU
- First reported in China in February 1957
- Spread throughout the globe within six months
- Infected 40-50% of the world's population
- Killed more than 1m people
1968 HONG KONG FLU
- Broke out in China in July 1968
- Spread throughout rest of world by mid-1969
- Fatalities ranged from 1-3m
- More than 30,000 deaths in the UK
Seasonal influenza occurs on a regular basis and by itself is estimated by the World Health Organisation to kill anywhere between 250,000 and 500,000 people each year.
In this list of the world's top ten causes of death, fatalities from lower respiratory infections which includes deaths attributed to influenza, ranks third highest.
| TOP TEN CAUSES OF DEATH WORLDWIDE 2004
|Coronary heart disease
|Stroke and other cerebrovascular diseases
|Lower respiratory infections
|Chronic obstructive pulmonary disease
|Trachea, bronchus, lung cancers
|Road traffic accidents
|Prematurity and low birth weight
The Health Protection Agency says figures for England suggest the number of people going to their GP with flu-like symptoms has now exceeded the seasonal average.
But figures are still well below the winter peak in 1999 when a particularly virulent strain of "Sydney" flu caused thousands of deaths.