Something horrible I learned from Shirley Saunders' book.
Roundworms live in the intestines of dogs and cats. They pass their eggs into the feces. These tiny eggs can survive in the soil for months.
You might not see any dog or cat plop on the ground on which you walk, but potentially you are picking up Roundworm eggs on your shoes.
If you allow shoes in your house, Roundworm eggs may be introduced onto the same floor on which your children play. It just takes your child one mouth to hand contact for her to become infected. Potentially she could develope lung problems as a result of infection.
Just take your shoes off and ask others to do the same. Its not rocket science.