THE CLASSIC STAGES OF CLINICAL INFECTIONS

As the body of the host responds to the invasive and toxigenic activities of a parasite, it passes through four distinct phases of infection and disease: the incubation period, the prodromium, the period of invasive, and the convalescent period.

            The incubation period the time form initial contact with the infectious agent to the appearance of the first symptoms. During the incubation period, the agent is multiplying at the portal of entry but has not yet caused enough damage to elicit symptoms. Although this period is relatively well defined and predictable for each microorganism, it does vary according to host resistance, degree of virulence, and distance between the target organ and the portal of entry (the farther apart, the longer the incubation period). Overall, an incubation period can range from several hours in pneumonic plaque to several years in leprosy. The majority of infections, however, have incubation periods ranging between 2 and 30 days.

            The earliest notable symptoms of infection appear as a vague feeling of discomfort, such as head muscle aches, fatigue, upset stomach, and general malaise. This short period (1-2 days) is known as the prodromal stage. The infectious agent next enters a period of invasion, during which it multiplies at high levels, exhibits its greatest toxicity, and becomes well established in its target tissue. This period is often marked by fever and other prominent and more specific signs and symptoms, which can include cough, rashes, diarrhea, loss of muscle controls, swelling, jaundice, discharge of exudates, or severe pain, depending on the particular infection. The length of this period is extremely variable.

            As the patient begins to respond to the infection, the symptoms decline- sometimes dramatically, other times slowly. During the recovery that follows, called the convalescent period, the patient’s strength and health gradually return owing to the healing nature of the immune response. An infection that results in death is called terminal. This term is particularly applicable if the infection is the immediate cause of death in a patient already suffering from a degenerative disease or cancer. Thus, an alcoholic might succumb to terminal pneumonia, or an AIDS patient will actually die from a secondary infection.

            The transmissibility of the microbe during these four stages must be considered on an individual basis. A few agents are release mostly during incubation (measles, for example); many are released during the invasive period (shigella); and others can be transmitted during all of these periods (hepatitis B).