Opportunistic, infections are often caused by naturally occurring organisms that reside in most individuals. These organisms typically are kept in check by a healthy immune system, and many of them—for example, certain types of digestion-aiding bacteria in the intestine—are actually beneficial to normal body function. Disturbances in the integrity of the gastrointestinal tract, often the result of chemotherapy or radiation therapy, can introduce intestinal bacteria into the bloodstream, which can ultimately lead to life-threatening infection. Prolonged therapy with antibiotics can suppress the normal, resident bacteria that tend to keep fungal organisms like Candida in check, and the fungi can initiate a potentially dangerous infection.

The definition of an opportunistic infection should also include those infections caused by organisms that are normally pathogenic in healthy hosts but that are more common or induce more severe infections in the immune-impaired host. For example, although a nonimmune, healthy person who comes into contact with varicella virus might develop, and recover from, chickenpox, a person with an impaired immune system has a good chance of dying from the infection.

''Reactivated" infections, another type of opportunistic infection, occur in people who were previously infected with an organism that the body was able to suppress but not eliminate completely. When the immune systems of these individuals weaken, the circulating organism has a chance to cause disease again, or, in many cases, for the first time. For example, an estimated 80 percent of Americans are infected with cytomegalovirus (CMV), a herpesvirus (National Institute of Allergy and Infectious Diseases, 1991b). The virus typically does not produce serious illness in healthy adults, but for transplant recipients, who receive immunosuppressive drugs to keep them from rejecting foreign tissue, CMV can be a life-threatening complication.

Tuberculosis (TB) is another example of an infection that can be reactivated during immunosuppression. The causative agent of TB, Mycobacterium tuberculosis, usually persists in the body long after primary infection. Although infection with this bacterium in a previously unexposed person is usually self-limiting, reactivated TB, which can occur years later, can cause life-threatening lung disease. In recent years, TB has stricken HIV-infected individuals with alarming severity, causing a rapidly disseminated disease involving organs throughout the body.

After declining steadily since the 1950s, the incidence of TB in the United States has recently begun to climb. Since 1986, reported cases have increased 16 percent (see Figure 2-2) (Snider and Roper, 1992). This trend is largely attributable to cases of TB among those infected with HIV. TB is also occurring with greater frequency among immigrants and refugees, substance abusers, the homeless, the medically underserved, and the elderly. The majority of the increase has been among racial and ethnic minorities (especially blacks and Hispanics), children and young adults, and immigrants and refugees.

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