Influenza in 1918: Winter seasonality · 473

A winter respiratory virus of drift and rare shift — surveillance and vaccines, not a cartoon villain. Influenza viruses are Orthomyxoviridae. Types A and B drive most human seasonal disease; A also has animal reservoirs. Influenza A has eight RNA segments, which is why reassortment can create new HA/NA subtype combinations.

The 1918 H1N1 pandemic killed tens of millions worldwide — a benchmark for later pandemic planning. WHO-linked meetings pick Northern Hemisphere vaccine strains in February and Southern ones in September. Symptoms often start about 1–4 days after exposure. People can shed virus before they feel ill.

In recent decades, seasonal A viruses in people have been mainly H1N1 and H3N2, plus influenza B lineages. Influenza is a seasonal respiratory virus that also, rarely, rearranges into pandemic subtypes. Public-health work is strain surveillance, vaccines updated twice a year, and ordinary indoor-air caution in winter.