Influenza Virologic Surveillance Landscape illustrating the processes and the flow of specimens and test data through 5 tiers of testing activity. CDC, Centers for Disease Control and Prevention; ILI, influenza-like illness; ILINet, Influenza-Like Illness Surveillance Program; NGS, next-generation sequencing; NIRC, National Influenza Reference Center; NREVSS, National Respiratory and Enteric Virus Surveillance System; PHL, public health laboratory; RIDT, rapid influenza diagnostic test. *Situation of concern: epidemiologic factors indicating outbreak, potential for severe disease, resistant infection, or possible novel virus infection.