Final answer:
Before the development of the electron microscope, scientists knew viruses must exist because they demonstrated the presence of infectious agents smaller than bacteria, referred to as "filterable agents," like those discovered by Frederick Twort.
Step-by-step explanation:
Before the first electron micrograph of a virus was produced in 1939, scientists had inferred the existence of viruses. They knew about viruses because they could demonstrate that particles smaller than bacteria could cause disease. These particles were referred to as "filterable agents" since they could pass through filters that were known to trap bacteria, indicating that they were much smaller. An example of early viral discovery is the work of English bacteriologist Frederick Twort in 1915, who observed tiny clear spots within bacterial colonies and hypothesized that an agent was killing the bacteria, which led to the discovery of bacteriophages, viruses that attack bacteria.
Virus discovery was possible due to indirect evidence and scientific deduction even before they were visible under microscopes. This evidence came from experiments showing that these agents could cause infections and were not visible nor retainable by methods that could easily capture bacteria.