These things sound outlandish even to me sometimes, but I just enjoy thinking them up really. Strange hobby, I know. Some of my theories I truly believe, this one I really don’t want to.
A microscopic parasite created in a lab that is easily transmitted airborne and will spawn in any mucous membrane, including ears.
Make this parasite have an immune suppressor as well as a severe seratonin inhibitor. Population control on several spectrums.