Germs

Lillian and Grayson have had colds for the last few days. Lillian has a nasty cough. I told her to cover her mouth, when she is coughing.

Lillian: Why, mama?
Mama: So you don't get germs everywhere?
Her eyes got wide.
Lillian: Oh no, there are germs all over my room!

This has caused many questions about germs. Are they in my belly? Why can't I see them? Are they everywhere? I realized quickly I am in over my head. The explanation of germs has totally stumped me.