Illustration of influenza virus amongst red blood cells in the bloodstream. There are three types of virus that all have a nearly identical morphology : Types A and B have two types of envelope glycoprotein or viral surface proteins (neuraminidase and haemagglutinin). Type C virus doesn't have surface neuraminidase. 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.