Antibodies and flu viruses. Illustration of antibodies (pink) attacking influenza virus particles (brown). The antibodies bind to the hemagglutinin protein, which contains the antigenic site of the virus. This binding by the antibodies prevents the virus particles from binding to the surface of human cells. Influenza is in the orthomyxovirus group of viruses. It infects the respiratory tract and causes fever, aching muscles, a sore throat and feelings of fatigue. New strains of the virus develop rapidly, and can cause epidemics.