Of the CDC and USDA select agents discussed, how many diseases are not zoonotic?

• A. Zero
• B. One
• C. Two
• D. Three

Answer: (C)

Zoonosis means a disease can move from animals to people. Among the CDC and USDA select agents discussed, most have some involvement of animals in their transmission or maintenance in nature. But two diseases on the list do not involve animals in their transmission; they are human-only pathogens in their infectious cycle. A familiar example is variola virus (smallpox), which spreads only from person to person and has no animal reservoir. The other non-zoonotic disease on the list shares this human-only transmission pattern. Because there are two such diseases, the correct count is two. This distinction matters for how we approach surveillance and control, since zoonotic agents require monitoring in both animal and human populations, while non-zoonotic, human-only diseases require focus on human-to-human transmission dynamics.