Being too clean leads to allergies.
OCD
Not proven. The “hygiene hypothesis” as a cause of allergies is appealing but too simple, making it incomplete and misleading. Research points strongly toward the most common factors in allergies as being overuse of antibiotics, poor diet, poor gut microbiome, and individual immunity. The best advice for parents is to keep the house and kids clean, but feed them well, get them outdoors, and let them have pets. And make them wash their hands frequently and assiduously.