Immunity refers to the body's ability to keep illness away. If you have a strong immune system, then your body is able to keep itself healthy by fighting illness.
Therefore, a compromised immune system means a body that does not have good immunity - a body that does not have the ability to defend against illness or medical challenges. If you have heart disease, diabetes, cancer, or any chronic illness, or if you have an acute illness that seems to last for a long period of time, your body system may be consider compromised.
If you have a compromised immune system (also called immunosupressed) then you may be at risk for contracting an infectious illness that can make you very sick, or will cause you to die.
The CDC defines high-risk more specifically as adults and children who have chronic pulmonary, cardiovascular, hepatic, hematological, neurologic, neuromuscular, or metabolic disorders; Adults and children who have immunosuppression (including immunosuppression caused by medications or by HIV); Residents of nursing homes and other chronic-care facilities.
Within patient empowerment, we consider infectious public health problems such as the flu (seasonal flu or H1N1 swine flu) that will require flu shots, or we look at problems with resistant infections such as those that are hospital acquired, like MRSA, C.Diff and others. Then we look at possible ways of keeping those problems at bay, to protect someone with a compromised immune system.