Coronaviruses are a group of related RNA viruses that cause diseases in mammals and birds. In humans and birds, they cause respiratory tract infections that can range from mild to lethal. Mild illnesses in humans include some cases of the common cold (which is also caused by other viruses, predominantly rhinoviruses), while more lethal varieties can cause SARS, MERS, and COVID-19. In cows and pigs they cause diarrhea, while in mice they cause hepatitis and encephalomyelitis. There are as yet no vaccines or antiviral drugs to prevent or treat human coronavirus infections.
Coronaviruses constitute the subfamily Orthocoronavirinae, in the family Coronaviridae, order Nidovirales, and realm Riboviria. They are enveloped viruses with a positive-sense single-stranded RNA genome and a nucleocapsid of helical symmetry. The genome size of coronaviruses ranges from approximately 26 to 32 kilobases, one of the largest among RNA viruses. They have characteristic club-shaped spikes that project from their surface, which in electron micrographs create an image reminiscent of the solar corona, from which their name derives.
Orthomyxoviridae (ὀρθός, orthós, Greek for "straight"; μύξα, mýxa, Greek for "mucus") is a family of negative-sense RNA viruses. It includes seven genera: Alphainfluenzavirus, Betainfluenzavirus, Deltainfluenzavirus, Gammainfluenzavirus, Isavirus, Thogotovirus, and Quaranjavirus. The first four genera contain viruses that cause influenza in vertebrates, including humans, birds (see also avian influenza), and other mammals. Isaviruses infect salmon; the thogotoviruses are arboviruses, infecting vertebrates and invertebrates, such as ticks and mosquitoes. The Quaranjaviruses are also arboviruses, infecting arthropods as well as birds.
The four genera of Influenza virus that infect vertebrates, which are identified by antigenic differences in their nucleoprotein and matrix protein, are as follows:
Alphainfluenzavirus infects humans, other mammals, and birds, and causes all flu pandemics
Betainfluenzavirus infects humans and seals
Deltainfluenzavirus infects pigs and cattle
Gammainfluenzavirus infects humans, pigs, and dogs.