Immune systems stay healthy when they are tested a bit. Obviously you need to cook food well and was food that might have been handled needs washing but sterilized..... noooooooo
What food are we talking about? Food in jars and cans needs heat treating ....otherwise it would "go off"....and cause all sorts of health problems....so, yes, it does assist us in staying healthy!
We have always lived a really simple fresh organic life where we eat stuff that's just come out of the soil, ha bird poo on eggs etc. We cook meat well, and make sure areas used to prepare food is very clean but as to making food as sterile as possible I wouldn't dream of it, we pick apples off the tree and eat them without washing them for example, and we are never tummy buggy. If you are canning things in jars etc then as gingeebee says things need to be spotless but otherwise no.
Unless you are immunosuppressed or immunocompromised, I don't see how it can be advantageous to live life in a sterile bubble.
There is a type of childhood cancer (of the blood) which experts feel is possibly due to a lack of exposure to bacteria and bugs in early development, which prevents them forming a robust immune system.
If the urchin's dummy falls on the floor I don't run like loony to the pantry looking for the sterilising fluid - I suck it like they used to do in my grandmother's day and pop it back in his mouth.