Orthomyxoviridae

Influenza A and B have eight RNA segments; influenza C has seven. Because they are negative-sense, the virion must carry its own RNA-dependent RNA polymerase. Two surface glycoproteins define serotype and drive pathogenesis: hemagglutinin binds sialic acid receptors to mediate attachment and entry, and neuraminidase cleaves sialic acid to release progeny virions. Antigenic drift is the gradual accumulation of point mutations in these genes and causes the seasonal epidemics that necessitate annual reimmunization; antigenic shift is the wholesale exchange of genome segments when two different influenza A strains coinfect one host, classically a pig acting as a mixing vessel, and it produces the novel subtypes responsible for pandemics. Only influenza A undergoes shift and only influenza A has a significant animal reservoir. A dreaded complication of influenza is secondary bacterial pneumonia, most often Staphylococcus aureus or Streptococcus pneumoniae.