Here's why:
* Concrete nouns refer to things that are tangible, meaning you can see, touch, smell, taste, or hear them. Germs, even though microscopic, are physical organisms that exist in the real world.
* Abstract nouns refer to ideas, concepts, qualities, or feelings that you can't physically interact with.
Since you can (in theory) observe germs under a microscope, they fall into the category of concrete nouns.