Research has revealed that the 1918 pandemic was so lethal due to group of three genes that permits the virus to attack lungs, causing pneumonia. The genes allowed the virus to reproduce in lung tissue.

The three genes -- called PA, PB1, PB2 an NP gene, made modern seasonal flu proved fatal for ferrets in similar fashion as the 1918 flu.

A US-Japanese research team has succeeded in isolating the three genes that has solved the mystery of 1918 Spanish flu.

The pandemic is believed to be the most lethal infectious disease in history. 20 and 50 million people died in it.

Experts apprehend that a pandemic of influenza will almost certainly strike again. However, they are not sure about its type and timing. But they do suspect it to be H5N1 avian influenza virus.

In majority of cases flu causes an upper respiratory infection which affects the nose and throat. It causes fever, muscle aches and weakness. But in some cases it degenerates into pneumonia.

The research findings were published an issue of the Proceedings of the National Academy of Sciences. The study also indicates that mutations might turn ordinary flu into a dangerous pandemic strain.