Final answer:
Scientists inferred the existence of viruses before seeing them under the electron microscope by observing the impact of particles smaller than bacteria on organisms and the spread of infectious diseases.
Step-by-step explanation:
Scientists were able to infer the existence of viruses before they were seen under the electron microscope. They observed that particles smaller than bacteria caused disease, which they called "filterable agents." Although they couldn't see viruses directly, they knew they must exist based on their impact on organisms and the spread of infectious diseases.