Which of the following is listed as a category of infectious disease pathogens?

• A. Bacterial
• B. Protozoa
• C. Algae
• D. Yeast

Answer: (A)

Pathogens are grouped by the type of organism they are, and bacteria form a major category of infectious agents. Bacteria are single-celled organisms with simple cell structure that can cause a wide range of infections in humans, which is why they’re listed as a broad category in many practice materials. The other options refer to organisms that can be involved in infections as well, but they belong to different categories (protozoa is a separate group; yeast are fungi; algae aren’t typically treated as a main pathogen category in standard infectious-disease classifications). So identifying bacteria as the broad category reflects the common way these pathogens are organized and discussed in medical contexts.