The secrets of why the 1918 flu pandemic was such a deadly one have been revealed by team of Japanese and American scientists.
The team discovered that the reason it was so deadly was because a group of three genes allows the virus to invade the lungs and cause pneumonia.
They discovered the three genes by mixing samples of the 1918 influenza strain with modern seasonal flu viruses and they believe their findings may help in the development of new flu drugs and also indicate mutations that might turn ordinary flu into a dangerous pandemic strain.
For their research virologist Dr. Yoshihiro Kawaoka of the University of Wisconsin and colleagues at the Universities of Kobe and Tokyo in Japan used ferrets, which develop flu in ways very similar to humans.
As a rule flu causes an upper respiratory infection affecting the nose and throat, as well as so-called systemic illness causing fever, muscle aches and weakness but some people become seriously ill and develop pneumonia caused by bacteria or by the flu.
It is during pandemics that new and more dangerous flu strain emerge and the researchers say the 1918 influenza pandemic was the most devastating outbreak of infectious disease in human history causing about 50 million deaths worldwide.
Kawaoka's team say that pandemic killed 2.5% of victims, compared to fewer than 1% during most annual flu epidemics and autopsies revealed that many of the victims, often otherwise healthy young adults, died of severe pneumonia.
Kawaoka's team wanted to find out why the 1918 flu caused severe pneumonia so they carefully substituted single genes from the 1918 virus into modern flu viruses and they behave like common flu, infecting only the upper respiratory tract.