What is the key difference between a reservoir and a source of infection?

• A. A reservoir refers to transmission route; a source refers to the host.
• B. A reservoir is the organism or environment in which a pathogen persists; a source is the immediate origin of infection for a case (for example, contaminated water).
• C. A reservoir is the immediate origin of infection for a case; a source is the environment where the pathogen persists.
• D. A reservoir and a source are the same concept.

Answer: (B)

The difference being tested is between where a pathogen persists over time and the immediate cause that leads to an infection in a person. The reservoir is the organism or environment in which the pathogen resides and can survive or multiply, sustaining itself in the population or in nature. It isn’t tied to a single exposure event; it’s about ongoing maintenance of the pathogen. The source of infection, on the other hand, is the immediate origin of infection for a particular case—the specific exposure that directly transferred the pathogen to the person, such as a contaminated water source, a tainted food item, or a used instrument.

For example, a cholera-causing bacterium may persist in a river or other aquatic environment, which serves as the reservoir. The contaminated water from a specific well that a person drank would be the source of infection for that person. This distinction—persistence in the reservoir versus the instantaneous exposure that causes a given case—is what sets these concepts apart.