Microbiology is not the exact science many think of it as. Peering through a microscope does not always reveal discrete microbial forms that can be easily classified within a clearly delineated taxonomy. Bacteria can change forms, often in ways that make recognition difficult. Some take on round shapes, others look like rods, while still others are curved or even tightly spiraled. Over years of trial and error, chemical solutions were created that could stain certain bacteria in order to make them more easily identifiable. Because not all bacteria would stain, this became an attribute in and of itself that could be used to help determine what someone was looking at.
[Edward] Rosenow, having studied bacterial infections within the brain for many years, was keenly aware of this. He knew that bacteria could shift and slide around in ways that were difficult to detect—sometimes becoming so small they could apparently pass through their most delicate filters, an ability which led many scientists to refer to them as viruses. Nowadays, we differentiate the two because bacteria can replicate on their own, whereas viruses require a host—another living cell. This crucial difference was not understood at the time, and as such, the ability to pass through Berkeland filters was what determined if something was classified as a virus or bacteria—it was their apparent size, not their mode of reproduction.
Years later, antibiotics would exploit this characteristic of bacteria to great effect. The penicillin family of drugs work specifically by causing bacteria to lose their cell wall—a dramatic event which effectively kills them. Unfortunately, some bacteria are not bothered the least bit by this process and, in fact, can begin replicating inside other living cells—just like viruses—a process with ramifications so nocuous as to deserve its own book. Because Rosenow sensed that some of the streptococcus bacteria he had cultured from poliomyelitis victims were transforming into a “virus” state, he sought the assistance of a fellow scientist who had built a unique microscope that could see far beyond anything possible at the time.