A vaccine for autoimmune disorders
When our society got rich, we started living much more sterile lives, and a whole bunch of diseases cropped up which are autoimmune disorders. These range from allergies to Chrohn's Disease, which destroys the bowel. Many of these syndromes did not exist in the pre-sterile world. (Not all autoimmune disorders are this way, of course.)
So some parents have become aware that you need to let your kids play in the muck, and with animals, and get exposed to diseases and parasites so your immune system grows up as you grow up. Otherwise, with nothing to do, it can attack you.
But parents are protective. They are not going to deliberately expose their children to parasites. But there are treatments that have been developed for sufferers of these diseases that give them safe alternatives that their immune system can fight. For example, the spores of parisites that infect other animals, but not humans.
So we need to develop a regime of "vaccines" against autoimmune disorders. A regimen of safe infectious agents that will put the immune system through the paces it expects to go through in the natural world, but which won't cause damage. And we need to expose children, and possibly even adults, to them through their lives.