You have a cold and sinus trouble again. Children in preschool or with preschool siblings get their virus introduction and ear infections very early.
Little children are hopelessly unsanitary.
Their snotty noses run, they put everything in their mouths, and they get secretions on their hands and contaminate whatever they touch. A small dose of virus can give you immunity with few symptoms but a big dose can go down in your chest and cause trouble. It is important not to get big doses. Grandparents with asthma or other chest trouble probably should not baby-sit small children with infections. The rest of us usually help when needed.