Which of the following correctly describes a susceptible host?

A susceptible host is defined as an individual who is vulnerable to infection due to various factors that impair their ability to resist pathogens. This includes people who may lack immunity, either due to having never been exposed to a certain pathogen prior, or having weak immunity for any other reason.

In the context of the choices presented, a new human host at risk for infection aligns most closely with this definition. This type of individual may not have developed specific immunities to various infections, especially if they are a newcomer to an environment where certain pathogens are present.

In contrast, the other options depict individuals who generally possess stronger defenses against infections. For instance, someone who is currently healthy is not necessarily at risk for infection and may have a functioning immune response. A person who has been vaccinated typically has some level of immunity against specific diseases, thus reducing their susceptibility. Similarly, a person with a strong immune system is less likely to be classified as susceptible since they are better equipped to fend off infections. Therefore, the correct answer highlights the inherent vulnerability of individuals new to exposure, making them more likely hosts for infections.