Negative-stained Transmission Electron Micrograph (TEM) revealing the presence of a number of influenza virus virions. This virus is a Orthomyxoviridae virus family member. The Orthomyxoviruses are a family of RNA viruses that includes seven genera: Influenza virus A, Influenza virus B, Influenza virus C, Influenza virus D, Isavirus, Thogotovirus and Quaranjavirus. The first three genera contain viruses that cause influenza in vertebrates, including birds (see also avian influenza), humans, and other mammals. Influenza A and B viruses cause seasonal epidemics of disease almost every winter in the United States. Influenza C infections cause a mild respiratory illness and are not thought to cause epidemics. Magnification: unknown.