부산대학교 도서관

Infections that cause disease and occur typically when the host's ability to fight back (via the immune system) is impaired. Opportunistic infections are types of infections that arise more often and with more serious effects in individuals with debilitated immune systems. Some microorganisms can be both opportunistic and nonopportunistic. An example is the bacterium Mycobacterium tuberculosis (Fig. 1). A healthy person who is exposed to a sufficient number of M. tuberculosis bacteria can develop pulmonary tuberculosis and become quite ill. However, most people with normal immune systems who are exposed to tuberculosis never become clinically ill, even though they may become infected. In this case, the body's immune defenses are capable of isolating the M. tuberculosis and preventing it from spreading throughout the body and causing symptoms or clinical disease. However, if an individual infected with tuberculosis also has an immune deficiency, such as acquired immunodeficiency syndrome (AIDS), the disease process is much different. Individuals with AIDS who are exposed to an infected person with active tuberculosis are much more likely to become ill, have more widespread disease, infect others, and die from the disease. Fortunately, if the diagnosis is made in time, antituberculous medications work effectively regardless of the underlying condition.