Highly pathogenic H5N1 avian influenza virus can infect humans and is currently the most deadly influenza virus that has crossed the species barrier. As of December 2007, the spread of H5N1 virus from human to human has been rare. Nobody can predict if H5N1 may cause a pandemic. However, the number of human cases is continuously increasing and changes in virulence and epidemiology have been detected. There are specific pathogenic features of H5N1 infection. In contrast to human-adapted influenza A strains, H5N1 preferentially infects cells of the lower respiratory tract and may spread to tissues outside the respiratory tract in humans. Moreover, H5N1 replication is prolonged in target organs and results in higher viral loads and increased tissue damage. These features will have to be considered for therapeutic protocols for H5N1 infection in humans. Rapid genetic and antigenic changes observed in H5N1 virus isolates represent a challenge for the development of vaccines. In the present review, current knowledge about epidemiology, virulence factors and pathology of H5N1 infections in humans are summarised and discussed. Moreover, the possible roles of anti-influenza drugs in the pandemic situation as well as the development of effective vaccines are subject of this overview
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