ELI5: Explain Like I'm 5

Hygiene hypothesis

The hygiene hypothesis is an idea that says if we are too clean, our immune systems might not develop correctly. It suggests that when we do not get exposed to bacteria, viruses, and other germs, our bodies don't learn to grow healthy and fight these germs properly. Being too clean might mean our immune system doesn't work as well as it should when it meets germs like colds, flus, and some allergies.