Influenza virus protein spikes. Computer artwork of part of the surface of a virus particle of the influenza (flu) virus. These spikes (formed by the surface proteins) allow the virus to attach to its host cell. Influenza belongs to the orthomyxovirus group of viruses. It contains a core of RNA (ribonucleic acid) genetic material inside a protein coat. Influenza is an infection of the respiratory tract. It causes fever, aching muscles, a sore throat and weakness. New strains of the virus develop rapidly, and can cause epidemics.