Imagine an kid in school first hearing about germ theory. The conversation might go something like this:

Teacher: Many diseases like the common cold are spread by germs, when one infected person contacts another.

Student: But I got a cold a few weeks ago, and I never touch anyone except my family members. And none of them were sick.

Teacher: You don't need to actually touch someone. Sometimes it can spread through mucus droplets in the air.

Student: And one time I was camping in the woods for a month, and then I got a cold, even though I hadn't been around anybody.

Teacher: If it was spring, you might have gotten allergies. Allergies can feel a lot like a cold, but they aren't spread by germs.

Student: It was fall.

Matt Singer Talks Movies TV and his Siskel Ebert Book