Scott Alexander at Astral Codex Ten:
Flu is a disease caused by a family of related influenza viruses. Pandemic flu is always caused by the influenza A virus. Influenza A has two surface antigen proteins, hemagglutinin (18 flavors) and neuraminidase (11 flavors). A particular flu strain is named after which flavors of these two proteins it has – for example, H3N2, or H5N1.
Influenza A evolved in birds, and stayed there for at least thousands of years. It crossed to humans later, maybe during historic times – different sources give suggest dates as early as 500 BC or as late as 1500 AD. It probably crossed over multiple times. Maybe it died out in humans after some crossovers, stuck around in birds, and crossed over from birds to humans again later.
During historic times, the flu has followed a pattern of big pandemics once every few decades, plus small seasonal epidemics each winter. The big pandemics happen when a new strain of flu crosses from animals into humans. Then the new strain sticks around, undergoes normal gradual mutation, and once a year immune response decays enough / mutations accumulate enough to cause another small seasonal epidemic (Why is this synced to the calendar year? See here for more).
More here.
Enjoying the content on 3QD? Help keep us going by donating now.