Which scientist contributed to the early concept of viruses by showing viruses are filterable and require host cells for replication?

• A. Beijerinck (and Ivanovsky)
• B. Pasteur
• C. Koch
• D. Winogradsky

Answer: (A)

The idea being tested is that some infectious agents are smaller than bacteria and must reproduce inside living cells. Ivanovsky showed that the agent causing tobacco mosaic disease could pass through a filter that trapped bacteria, indicating it was a filterable entity. Beijerinck expanded on this by demonstrating that the filtrate remained infectious and could multiply only within living plant tissue, and he coined the term virus to describe this new kind of pathogen. Together, their work established viruses as a distinct class of infectious agents that require host cells to replicate, not free-living organisms. Other foundational microbiologists—Pasteur, Koch, and Winogradsky—made pivotal contributions to germ theory and bacterial biology, but they did not identify the filterable, host-dependent nature of viruses.