As long as there has been life on earth, there have been germs. But because germs are invisible to the eye, it took the invention of the microscope to see that germs are all around us. And it took the genius of Louis Pasteur to recognize the disease-causing and curing effects of bacteria and viruses—a discovery that has paved the way for modern medicine.
A nice little introduction to the history of microbiology with emphasis on the germ theory of disease and the discovery of viruses. A good book, with material well-covered elsewhere (e.g., in Asimov’s New Guide to Science) for adults.