A team of Japanese and Vietnamese researchers have discovered a mutation in the avian influenza virus that may cause the disease to spread from person to person, the Japanese media reported. At present, the H5N1 virus that causes bird flu infects human only by contact with diseased birds.

The Yomiuri Shimbun reported on Monday that the findings of the team, led by Prof. Yoshihiro Kawaoka of Tokyo University's Institute of Medical Science, was published on a web-based science journal on Friday.

In an experiment, the team found that a certain amino acid from a protein of H5N1 virus samples tend to grow in a human's nasal membrane. They artificially converted the amino acid into another type of amino acid called lysine and transmitted the virus and the lysine to mice.

Three days later, the number of viruses in the mice's nasal membrane was more than 10,000 times bigger than in mice infected with ordinary avian flu

The Kyodo News said the mutation allows the H5N1 to live in the cooler temperature of the throat and nose, where the virus is easily transmitted among humans through sneezing and coughing.