How does an individual become a reservoir host?

An individual becomes a reservoir host primarily by becoming infected with a pathogen and subsequently transferring that pathogen to others. In this context, a reservoir host is an organism that harbors a pathogen, allowing it to live and multiply, and often facilitating its transmission to other susceptible hosts. This can occur without necessarily causing disease in the reservoir host, which can go on to shed the pathogen into the environment or directly to another individual.

Consuming contaminated food can lead to infection, but it does not inherently make someone a reservoir unless they become infected and carry the pathogen afterward. Vaccination is a preventive measure aimed at protecting individuals from infections and does not create a reservoir. Similarly, having a healthy immune system usually helps resist infections rather than contribute to being a reservoir host. Thus, the correct choice emphasizes the role of active infection and transmission in establishing a reservoir host.