Influenza-A bird viruses are named based on the alphanumeric reference of “H” and “N”. Influenza-A subtypes are referred to as H1-H18, and N1-N10; thus, influenza has 180 potential sub-types of H and N. Some influenza-A strains are known to infect humans, and are transmitted person-to-person; and the dominant strains today are H1N1 and H3N2. Other strains can rarely be transmitted from animals-to-people, such as H5N1, and cause severe disease, but have limited potential for person-to-person transmission.

Many influenza-A strains infect a variety of animals, which creates intermediate hosts capable of causing a “re-assortment” or evolution of strains. The H17 and H18 influenza viruses were discovered in bats, but as yet have not been proven transmissible to humans. Bat viruses can be transmitted to dogs, pigs, horses, dogs, seals, and potentially other mammals such as pangolins, which can serve as a bridge to human infection.

The working theory of coronavirus is the virus originated in bats. Bats are heavily infected with a variety of viruses, and the evolution of the bat influenza viruses in co-infected animals is of significant concern. The genome of coronavirus most closely matches that of pathogens found in pangolins, and pangolins suffer from a similar frothy lung disease, which suggests coronavirus may have emerged from a re-assortment of a bat virus after transmission to pangolins, and become transmissible to humans.