What is a key characteristic of a susceptible host?

A key characteristic of a susceptible host is insufficient natural resistance to infection. This concept is grounded in understanding the host-pathogen interaction. When a host has weakened immune defenses or has not built up sufficient immunity to specific pathogens, they become more vulnerable to infections. Factors that can contribute to a host's susceptibility include age, pre-existing health conditions, malnutrition, and certain medications that suppress immune function.

The other options do not accurately represent characteristics of a susceptible host. For instance, high levels of physical activity can often enhance overall health and bolster the immune system, making a person less susceptible to infections. Frequent medical checkups typically indicate proactive health management, which can help identify vulnerabilities before they lead to increased susceptibility. Low exposure to pathogens generally leads to a lower likelihood of infection, contrasting with the state of susceptibility where the immune system is not adequately capable of responding to infections effectively. Thus, having insufficient natural resistance to infection firmly identifies the host as susceptible.