Rush Limbaugh, The Big Fat Idiot, imagines Vitamin D was known during the Spanish Flu Epidemic when it wasn't even first theorized until 1922

In the Face of COVID-19, We’re Not Acting at All Like Americans:

In the Spanish flu, ’17, ’18, ’19, 1917, much death. Do you know that there was not one mention of it by the president of the United States at the time, Woodrow Wilson? Never talked about it. There was no national policy to deal with it. There was no shutdown. There was just, “Hey, go outside, get some fresh air, stand in the sun as long as you can, get some vitamin D, feel better.”

Vitamin D:

In 1922, Elmer McCollum tested modified cod liver oil in which the vitamin A had been destroyed.[12] The modified oil cured the sick dogs, so McCollum concluded the factor in cod liver oil which cured rickets was distinct from vitamin A. He called it vitamin D because it was the fourth vitamin to be named.[194][195][196] It was not initially realized that, unlike other vitamins, vitamin D can be synthesised by humans through exposure to UV light.