Researchers found out the main cause of the deadly 1918 flu pandemic. Apparently, the main cause of the deadly nature of the disease was a group of three genes that permitted the virus to enter lungs causing pneumonia.

The team of Japanese researchers used mixed samples of the 1918 influenza strain with modern seasonal flu viruses and managed to find the three genes responsible for the flu pandemic. The findings of the research – published in Tuesday's issue of the Proceedings of the National Academy of Sciences – will most certainly be of great help in the future development of new flu drugs.

The research also pointed out that the certain mutation can turn an ordinary flu into a dangerous pandemic strain. Researchers at the University of Wisconsin and Universities of Kobe and Tokyo in Japan used ferrets in their research because the way the little animals develop flu is very similar to the way humans do.

Influenza can turn deadly, but the 1918 Spanish flu pandemic was something you see once in a thousand years. As much as 500 million people worldwide (1/3 of the world’s population) were infected with it back then and about 20 million to 30 million people died because of it.

The main difference between the ordinary flu and the 1918 one is that an ordinary influenza mainly affects the upper respiratory tract, while the Spanish flu pandemic also took root in the lungs.

The three genes responsible for the pandemic – called PA, PB1, and PB2 – affected a fourth one called NP. Researchers took those genes, subbed them into the K173 flu virus (which currently circulates in people) and tested the result on ferrets. This let the virus lodge in the animal’s lungs and the effects were similar to those of the 1918 flu.