HARVINDER SINGH / SCIENCE PHOTO LIBRARY HARVINDER SINGH / SCIENCE PHOTO LIBRARY
Illustration of influenza virus structure. There are three types of virus that all have a nearly identical morphology : Types A and B have two types of envelope glycoprotein (blue) or viral surface proteins (neuraminidase and haemagglutinin, pink and green). Type C virus doesn't have surface neuraminidase. The influenza virus is an RNA virus, that is, its genetic material is RNA (ribonucleic acid, shown as beaded coils). Flu usually causes lethargy, aching muscles, fever and nausea. Some strains, however, develop into far more serious forms, and epidemics in the past have killed millions.
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