the great flu pandemic of 1917-1919 was unusual in some respects. Older people , who were born prior to 1872, were immune to its effects, as they were "innoculated" in 1872 by the great horse flu pandemic of 1872. In 1872, all horses in the USA and canada were immobilized, either dying within 1 week or severely immobilized, by a flu, that likely originated by an avian flu carried by the migration of canadian geese. It spread throughout the USA exactly along the rail corridors where horses were transported by trains. It was so severe that horse drawn trolley cars and fire engines were pulled by immigrants for 1 yr , until the horse population recovered. The US cavalry's war against the Indians was halted for 1 yr as both sides didn't want to fight without horses. The analysis of the flu specimens from 1917 show it was based on the 1872 flu.
The 1917 great war effort included shipping over 1 million horses from the USA to europe on ships, and close to 50% of those horses died of "shipping fever", another name for a contagious horse flu. The 1917 flu likely originated in Kansas USA , and was distributed to US troops at a kansas camp and to europe via the horse shipments.
"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick