The hygiene hypothesis is the belief that overly sterile environments are causing children not to be able to build up their immune system properly. It was first proposed by a British physician named David Strachan in a paper called "Hay Fever, hygiene, and Household Size." This supposedly leads to an increase in allergies, asthma, sickness, and autoimmune diseases. This is a very interesting thing to look at. America is sterile. In every store, we have a hand sanitizer, antibacterial soap, and a myriad of very harsh chemicals for cleaning. The two countries with the highest autoimmune disease rate are the United States and the United Kingdom.
Now causation versus correlation. Just because the United States we have the highest autoimmune disease rate and we use a large variety of harsh chemicals. Now that could just be a coincidence. I am in no way saying that being clean or disinfecting is harming our country. But the hygiene hypothesis is saying that the overuse and over cleanliness, especially of our children, may be contributing to the fact that we do have a high rate of autoimmune disease. (1) There is even a study showing that the lack of certain microbes in the gut can increase your chance of developing type one diabeties. (2)
We know that children seek out sensory for their development. For example, my daughter is working on her balance. She will seek out any way she can to walk across a raised curb or log in order to get that practice and improve her gross motor skills. It is in our child's best interest to stand back and allow them to experience the world and develop their skills. Children do the same thing with their mouths. Now we know that children innately do what they need to develop their gross and fine motor skills, can the same be said for their immune system? Babies put everything in their mouths not only is that to use another sense to explore an object but also to enhance their immune system.
In the first few months, babies have a sort of protective immunity from their mother's placenta. After, when they are breastfed, they get natural immunity through their mother's milk. It is important to note that the mother's breast milk will change and produce antibodies for exactly what the baby needs. Baby formula does not have any of the antibodies found in breastmilk. So babies fed with formula are not necessarily in the same position to be as adventurous. If you think about it baby puts germs in mouth, baby puts mouth on mom, mom's breast milk creates antibodies to help teach the immune system to fight off the germs that were introduced to baby. This is a cycle that babies need to be getting in order to build up their immune system. (To be fair I am not farmiliar with formula feeding so I do not know if this same cycle will work, what I do know is formula does not provide babies with antibodies in the way breastmilk does.)
Now this is not me saying let your children eat anything and everything they find, just as I would not say let them do anything and everything they want. I sugggest letting children seek the sensory input of balancing on a raised curb, but I would not suggest allowing your child to walk on the edge of a cliff. In the same way I say let your kids explore taste within reason. A small amount of dirt or a not so clean toy will most likely not hurt your baby, but a rock may pose as a choking hazard or some standing water may harbor parasites. It is all about letting your child explore their world within reason.