It seems sensible for parents to make sure their baby is in a clean place. Most parents do their best to keep dirt and bacteria away from their children. However, a scientist says dirt is important to make babies stronger. Dr Jack Gilbert studies 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 said: "It's fine to wash their hands if there's a…flu virus going around….but if a dog licks their face, that's not a bad thing."

Dr Gilbert said letting children get dirty was beneficial. The dirt helps small kids to build their immune system. Gilbert said children often get allergies because parents try too hard to clean everything that children use. He said parents now sterilize everything in the home too much. This makes children's immune systems very sensitive, which can lead to things like asthma and food allergies. Gilbert defended the "five-second rule". That is when you eat something that fell on the floor for fewer than five seconds. Gilbert says that is OK as long as the floor isn't really dirty.