A germ is a tiny life form that spreads disease. Germs are also small things that grow into larger things — whether animals, plants, or ideas.
Germ is not a technical term, but you've probably heard it used in reference to tiny organisms (such as bacteria) that cause disease. A germ is also any small structure, such as a fertilized egg, that grows into something larger. A seed that grows into a plant is a germ. In some cases, you can talk about ideas as germs. The inspiration for a book or movie can be described as the germ of the project. Germs are small, but they start big things.