Almost all illnesses are worth avoiding. Take for example, malaria. Dengue has 4-variants which can kill if you've already got any one of others the first time. COVID still hasnt been fully researched yet.
Back in the early 2000s there were "crunchy" moms on Livejournal who were mailing each other "pox boxes", which was a juice box a kid with chicken pox had licked I guess? It was absolutely insane and did not work.