The flu in this case is the epidemic, or rather pandemic, of Spanish influenza which struck Europe and America in the final stages of the first World War and raged for several months, killing off millions.
It was unlike any flu virus known to medicine, and it killed swiftly but horribly, carrying off young people in particular inside a few days. "Spanish influenza" is probably a misnomer, since the virus almost certainly did not originate there, though an outbreak in Northern Spain travelled across the Pyrenees into France. Soldiers of all armies suffered terribly, while certain nations weakened by war-time malnutrition also lost hugely. Gina Kolata's account of the disaster is written largely from an American viewpoint, and she has talked to many medical men and discusses at length the intensive research carried out in order to prevent a recurrence.