Answer:
Hygiene
Step-by-step explanation:
In medicine, the hygiene hypothesis states states that when children are in the early childhood age they are exposed to some microorganisms and viruses and bacteria that help them develop their immune system. According to this hypothesis, the fact that children are overprotected from viruses and bacterias when they are younger, doesn't help them develop and strengthen their immune systems and therefore, it leaves them more vulnerable to some diseases.
Therefore, some experts suggest a hygiene hypothesis in which contemporary children are overprotected from viruses and bacteria, which means they do not acquire infections and diseases that would strengthen their immune systems.