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A host factor is considered to be one of the intrinsic qualities of a person that helps determine that person's susceptibility to disease. Host factors may be thought of as risk factors for disease, but they form part of the patient's own physical or psychological constitution. This makes a host factor different from an environmental factor, as environmental factors for disease come from outside the patient, and can include such things as limited access to medical care or exposure to certain toxins. General health, nutritional state, psychological characteristics, quality and type of social ties, history of substance abuse, and race may all be considered to be host factors for disease, because these things influence or form part of the patient's mental and physical constitution.
Environmental factors can affect a person's constitution and change that person's host factors for disease. A person who recovers from a viral illness, for instance, may enjoy immunity to that illness in the future. This means that the person's host factor has changed to drastically reduce his vulnerability to the disease.
A disease's host factors may vary, depending on the nature of the illness. Some diseases may require more than one host factor in order to cause infection. With proper self-care practices, many people may be able to alter their host factors for disease, making themselves less susceptible to illness. Other host factors, such as race, age, and prior medical history, cannot be changed.
A generally good state of health can eliminate many host factors for disease. Many people receive preventative care to help reduce their host factors for illness. Obesity, for instance, is considered a primary host factor for many other conditions, including heart disease, stroke, and diabetes. Losing weight can help reduce or eliminate the host factor that may contribute to these disorders, lowering the person's chances of succumbing to them later in life.
Vaccinations are considered another way to eliminate host factors for disease, in this case, the host factor for susceptibility to a virus. The host factor for most viral infections is susceptibility to the virus. A person who has been immunized against a virus, or gained immunity by contracting and recovering from the virus, is considered to have what's known as a specific immunity to that virus. The person is not expected to contract that virus again in the future, because he is considered deficient in the host factor for that virus.