Two groups of scientists, in Japan and in Holland, have come up with explanations as to why human to human transmission of the H5N1 virus is highly unlikely. Unlike other known flu viruses which can be easily transmitted between people, the H5N1 virus can attach very hard to the cells in the nose, throat and upper airways, until getting to the cells in the lungs. This new data proves the fact that people need a close and heavy exposure to the fatal virus before reaching the lungs, where it causes extensive damage to the machinery of respiration. This explains why the virus cannot spread through coughs or sneezes. Hence, the virus would need several favorable mutations before having the capacity to cause a pandemic, said Yoshihiro Kawaoka, a virologist who took part in the study. Scientists already knew that those attacked by the current avian flu virus were infected in the lower lung. They also agree on the fact that, sooner or later, a pandemic will occur because at least one of the 16 types of flu virus in the animal world will change hosts, spreading among humans. It still remains unknown whether H5 will do that or one of the other viruses; previous pandemics have been caused by H1 type viruses (the 1918 pandemic), H2 (the 1957 Asian) or H3 (the Hong Kong flu of 1968). The H5 virus has been present in the human population since the late 1950's, but has never had the full set of mutations needed to set off a pandemic. Peter Palese, a virologist at the Mount Sinai School of Medicine, declared H5 infects people only when they are exposed to very large doses, for example when they sleep with chickens in the same room. "I feel strongly that H5 has been around in humans for a long time and never caused a pandemic," suggesting that this is not the virus which is likely to be the next pandemic.