The Orthomyxoviridae (orthos, Greek for "straight"; myxa, Greek for "mucus") are a family of RNA viruses. They include five genera: Influenzavirus A, Influenzavirus B, Influenzavirus C, Thogotovirus and Isavirus. The first three genera contain viruses that cause influenza in vertebrates, including birds (see also avian influenza), humans, and other mammals. Isaviruses infect salmon; thogotoviruses infect vertebrates and invertebrates, such as mosquitoes and sea lice.

The three genera of Influenzavirus can be told apart by the structure of their proteins. They infect vertebrates, as follows:

• Influenzavirus A cause of all flu pandemics and infect humans, other mammals and birds
• Influenzavirus B infect humans and seals
• Influenzavirus C infect humans and pigs