Tobacco mosaic virus helped establish viruses as a distinct class of infectious agents.

• A. It showed viruses are larger than bacteria
• B. It proved bacteria cause plant disease
• C. It helped establish that viruses are a distinct class of infectious agents
• D. It disproved germ theory

Answer: (C)

Understanding that tobacco mosaic virus helped establish viruses as a distinct class of infectious agents comes from how it demonstrated there are disease-causing entities that are smaller than bacteria and can pass through filters that trap bacteria, yet still require living cells to replicate. This pointed to a new kind of pathogen, different from bacteria, and led Beijerinck to describe a contagious agent that behaved like a distinct category—what we now call a virus. So the statement that it helped establish that viruses are a distinct class of infectious agents captures the key takeaway. It’s not about viruses being larger than bacteria, and it doesn’t claim bacteria cause plant disease—TMV is a virus, not a bacterium. It also doesn’t disprove germ theory; germ theory was expanded to include viruses as another type of disease-causing agent.