Compared with bacteria, viruses typically have which relative infectious dose?

• A. Higher infectious dose
• B. About the same infectious dose
• C. Lower infectious dose
• D. Infectious dose varies by pathogen

Answer: (C)

Viruses are able to establish infection with only a very small number of infectious units because they are obligate intracellular parasites that hijack host cells. A single virion can attach to a susceptible cell, enter, release its genome, and begin replication, triggering an infection that can spread quickly as new cells are recruited to produce more virus.

Bacteria, by contrast, often need to reach a larger initial population to overcome host defenses, compete with the normal microbiota, and sometimes survive harsh barriers like stomach acid or mucus before causing disease. Because of these differences in how infection is initiated and sustained, viruses typically require a lower infectious dose than bacteria. It’s worth noting that infectious dose does vary by pathogen and exposure route, but the general trend is a lower dose for viruses.