PATHOGENS

• Influenza viruses are enveloped, negative-sense, single-stranded RNA viruses in the Orthomyxoviridae family with segmented genomes.
  • Influenza A infects humans and numerous animal species and is classified by its surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA).
    • The principal seasonal human subtypes currently are A(H1N1)pdm09 and A(H3N2).
    • It circulates in animal reservoirs (birds, mammals), and its segmented genome permits reassortment (antigenic shift), conferring pandemic potential for influenza A.
    • The gradual accumulation of mutations (antigenic drift) drives seasonal epidemics and necessitates periodic updates to vaccine strains.
  • Influenza B predominantly infects humans and causes seasonal epidemics but not pandemics.
    • Unlike influenza A, it is not divided into H/N subtypes. Rather, lineages are described, e.g., B/Victoria.
    • B/Victoria is currently circulating; naturally occurring B/Yamagata has not been detected since 2020.
  • Influenza C infects humans and usually causes mild, sporadic upper-respiratory illness, particularly in children.
    • It is not an important cause of seasonal influenza epidemics, and is not included in routine influenza vaccines
    • It is not detected by standard clinical influenza A/B assays.
• Seasonal influenza A and B are among the most common respiratory viruses afflicting infants, children, and adults, along with SARS-CoV-2 and RSV.
  
  • 2025-2026 respiratory season, May 2026 estimates from the CDC, a season characterized as an usually rapid mid-winter surge driven by antigenically drifted influenza A (H3N2) known as subclade K.
    
    • Influenza cases in the U.S.: 32 million
      
      • Hospitalizations: 390,000
      • Influenza-associated deaths: ~24,000 with 191 reported pediatric deaths (Aug 2026 estimate)
      • Peak intensity for outpatient visits occurred just after the holidays, with influenza-like illness doctor visits peaking in December 2025.
• See the Avian Influenza module for details regarding these influenza viruses.