By 1920, the Spanish Flu had been around for three years and killed from 50 to 100 million people in the world. There were no vaccines, no antivirals or antiinflamitories back then except aspirin. Patients were being experimented on with different vaccines and treatments, very little worked at all.
However, at the beginning of the 20th century, music changed into black-inspired music called Jazz. A great admirer of this music was Al Jolson, a member of a minority group himself. He was a Jew. He was the Elvis Presley of his time and jazz was his rock and roll.
Al Jolson became a superstar in 1920 with Swanee and all his other songs. It was fun music and made people feel better. They forgot the suffering of WW1 and the epidemic.
There were lots of bad memories from the war, disease and medical treatments that didn't work in 1920, but people got powerful injections of feel good music from Al Jolson. The rest is history.