WASHINGTON — Researchers have found out what made the 1918 flu pandemic so deadly — a group of three genes that lets the virus invade the lungs and cause pneumonia. They mixed samples of the 1918 influenza strain with modern seasonal flu viruses to find the three genes and said their study might help in the development of new flu drugs. The discovery, published in Tuesday’s issue of the Proceedings of the National Academy of Sciences, could also point to mutations that might turn ordinary flu into a dangerous pandemic strain. 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. Usually 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. Sometimes bacteria cause the pneumonia and sometimes flu does it directly. During pandemics, such as in 1918, a new and more dangerous flu strain emerges. ‘The 1918 influenza pandemic was the most devastating outbreak of infectious disease in human history, […]