influenza viruses C and D are estimated to have diverged from a common ancestor around 8,000 years ago. Metatranscriptomics studies have also identified closely related "Influenza C and D-like" viruses in a number of amphibian species. Influenza A virus can infect a variety of animals as well as humans, and its natural host or reservoir is birds, whereas influenza viruses B, C, and D do not have animal reservoirs. Influenza viruses C and D are not as easily isolated so less information is known about these types, but studies show that they occur worldwide. This virus may be spread through respiratory droplets or by fomites (non-living material) due to its ability to survive on surfaces for short durations. Influenza viruses have a relatively short incubation period (lapse of time from exposure to pathogen to the appearance of symptoms) of 18–72 hours and infect the epithelial cells of the respiratory tract. In cell culture, influenza D virus has demonstrated an ability to replicate well at 37°C, the normal lung temperature, and can also replicate better and in more types of cells than the Type C virus. This study suggests that influenza D virus may be only a few genetic changes away from being able to invade the lower lung, even though the virus does not actively spread among humans and has a much slower mutation rate than the other influenza viruses. Structure and Variation Influenza viruses, like all viruses in the family Orthomyxoviridae, are enveloped RNA viruses with single stranded genomes. The antigens, matrix protein (M1) and nucleoprotein (NP), are used to determine if an influenza virus is type A, B, C, or D. The M1 protein is required for virus assembly and NP functions in transcription and replication. These viruses also contain proteins on the surface of the cell membrane called glycoproteins. Type A and B have two glycoproteins: hemagglutinin (HA) and neuraminidase (NA). Types C and D have only one glycoprotein: hemagglutinin-esterase fusion (HEF). These glycoproteins allow for attachment and fusion of viral and cellular membranes. Fusion of these membranes allows the viral proteins and genome to be released into the host cell, which then causes the infection. Types C and D are the only influenza viruses to express the enzyme esterase. This enzyme is similar to the enzyme neuraminidase produced by Types A and B in that they both function in destroying the host cell receptors. Glycoproteins may