No one was immune. Masked, sanitised and distanced, they caught it and subsequently spread it to everyone they came into contact with.
A plague. One to rival covid-19. There was no vaccine.
Big business and governments despaired. How could you sell more commercialism and propaganda to satisfied, happy people?
They tried in vain to stop it but she and all the other infected people continued to spread happiness. Kind words. Compliments. Joy.
And the world was a better place.