Immune fitness refers to the capacity of the body to respond to health challenges (such as infections) by activating an appropriate immune response, essential to maintain health, prevent and resolve disease, and improve quality of life [1]. Analogue to mental resilience, immune fitness is the ability to bounce back from physical events such as disease states and health complaints. Adequate immune fitness is thus crucial in determining longevity and plays an essential role from early life to healthy aging. In fact, reduced immune fitness may be the most important reason why patients visit their physician. As such, reduced immune fitness is an important health sign, and adequate assessment is therefore vital.