Influenza virus is a member of the family Orthomyxoviridae. There are five genera within this family of viruses, including Influenzavirus A, Influenzavirus B, Influenzavirus C, Thogotovirus, and Isavirus. Influenza is an enveloped, negative sense ribonucleic acid virus with a segmented genome that replicates in the nucleus of infected cells. The intracellular lifecycle of influenza virus begins with the attachment of the HA protein to cellular receptors containing sialic acid, a ubiquitous 9-carbon acidic monosaccharide that is found on the surface of cells and proteins. Although all three types of influenza viruses can infect humans, influenza A viruses can also infect a wide variety of other hosts, and it is this zoonotic potential that leads to the emergence of novel strains through continuous reassortment. The 1918 H1N1 influenza pandemic was marked by three distinct waves that occurred over an approximately 12-month period in 1918 and 1919.