The hosts' susceptibility to infection depends on several factors. The integrity of the skin and mucous membranes helps protect the body against microbial attacks. When the skin is altered, the chance of infection, limb loss, and even death increases.

The integrity and count of the white blood cells help the body resist pathogens and fight infection. When impaired, it reduces the body's resistance to pathogens. The acidic pH levels of the gastrointestinal, genitourinary tracts, and skin create a hostile environment for the colonization of pathogens. This means that a pH imbalance may predispose individuals to infections.

Natural or acquired immunity produces antibodies that resist infection. When an individual's immunity is affected, it lowers the body's ability to fight infection. An increased stress level may suppress the immune system, increasing susceptibility to illness.

Age, sex, race, and genetics influence susceptibility. Regarding age, the elderly are more likely to acquire an infectious disease. Levels of fatigue, nutritional, and general health status also affect the hosts' susceptibility. For instance, undernutrition causes immunosuppression, which later increases the chances of infection.

Pre-existing illnesses, previous or current treatments, and certain medications affect the hosts' susceptibility. For example, individuals with pre-existing diabetes are at greater risk of complications from COVID-19.