H5N1 avian influenza virus, coloured transmission electron micrograph (TEM). This subtype of avian influenza (bird flu) is a viral pathogen that causes a deadly disease in bird populations. Bird flu has killed large populations of birds in Southeast Asia and is spreading rapidly worldwide. It can spread to humans through close contact with infected birds or their faeces. There are no known cases of infection via human to human contact at this time. Concerns are that this subtype might mutate and turn in to a human influenza subtype that could cause a worldwide flu epidemic. Influenza viruses have the ability to swap genetic material with other species and subtypes. For these reasons influenza viruses are closely tracked. Magnification: x104,000 when shortest axis printed at 25 millimetres. Original image courtesy of the CDC.
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