How were viruses discovered?

At first, bacteria were thought to cause all diseases, as they were seen through microscopes. Louis Pasteur believed that some bacteria are simply very small and cannot be seen through a microscope. In 1892, biologist Dmitry Ivanovsky discovered viruses in a microscope, but he did not understand what it was, but mistook the virus for bacteria. Only a few years later, Martin Beijerink was able to determine that the infectious material that Ivanovsky isolated was nothing more than a new form of living organisms. This was a breakthrough in biology and medicine.