It seems like common sense to most parents to make sure their baby is always in a clean place.
Most parents do their best to keep dirt and bacteria away from their little ones.
However, a scientist says dirt is an important part of making babies stronger.
Dr Jack Gilbert from the University of Chicago studies the ecosystems of bacteria.
He did research into how dirt and bacteria affect children.
He published his research in a book called 'Dirt is Good: The Advantage of Germs for Your Child's Developing Immune System'.
He told America's NPR news station that: "It's fine to wash their hands if there's a…flu virus going around, but if they're interacting with a dog and the dog licks their face, that's not a bad thing."
Dr Gilbert wrote that letting children get dirty was largely beneficial.
Exposing small kids to dirt helps them to build their immune system.
Dr Gilbert even argues that children often get allergies because parents try to protect their kids too much and try too hard to clean everything that children use.
He said parents now over-sterilize everything in the home.
This causes children's immune systems to too sensitive, which can lead to things like asthma, eczema, and food allergies.
Gilbert even defended the "five-second rule". Many people think it is OK to eat something that fell on the floor for fewer than five seconds.
Gilbert says it is OK to eat something that fell on the floor as long as the floor isn't really dirty.