What term describes the body's capability of fighting off infections?

The term that describes the body's capability of fighting off infections is "immunity." Immunity refers specifically to the biological processes and mechanisms that provide resistance against pathogens such as viruses, bacteria, and other infectious agents. It can be innate, meaning the body’s initial, immediate response to any infectious agent, or adaptive, which involves specific responses tailored to particular pathogens, often resulting in the development of memory cells that enhance the body's ability to respond to future infections by the same pathogen.

Resistance, while closely related, is a broader term that can refer to any capability of the body to withstand the effects of disease but does not specifically denote the targeted mechanisms of immune protection. The terms response and defense also broadly describe the actions taken by the body in reaction to infections, but they do not encapsulate the full scope of the specialized immune functions involved in providing long-term protection against disease. Thus, "immunity" is the most accurate term in this context.