Which of the following is a characteristic that determines the suitability of pathogens?

• A. Viability
• B. Infectivity
• C. Size
• D. Color

Answer: (A)

Viability—the ability of a pathogen to stay alive and capable of replication—is the key factor in determining its suitability. A pathogen must be viable to invade host cells, replicate, and produce disease; if it’s non-viable, it cannot establish infection regardless of other traits. Infectivity relates to how readily a viable organism can cause infection once exposure occurs, but viability is the prerequisite that makes infectivity possible. Size and color don’t reliably indicate whether a microbe can cause disease; many viable, disease-causing organisms are small and colorless, while non-pathogenic organisms can vary in size and appearance.