Question

is being able to resist a pathogen that has previously infected the body.

Explanation:

The term that fits here is "Immunity" (specifically "Adaptive Immunity" or "Acquired Immunity" in the context of resisting a pathogen that previously infected the body, as the immune system remembers the pathogen and can mount a quicker response upon re - exposure). In the subfield of Biology (under Natural Science), the study of the immune system and how it responds to pathogens is a key area. So the word to fill in the blank is related to the body's ability to resist such a pathogen, which is "Immunity" (or more precisely "Acquired Immunity" or "Adaptive Immunity", but "Immunity" is a general term that fits here).

Answer:

Immunity