As the body of the host responds to the invasive and toxigenic activities of a pathogen, it passes through four distinct phases of infection and disease: the incubation period, the prodromal stage, the period of invasion, and the convalescent period (figure 1).
Fig1. Stages in the course of infection and disease. Dashed lines represent periods with a variable length.
The incubation period is the time from initial contact with the infectious agent (at the portal of entry) 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 plague to several years in Hansen’s disease (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 and 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 tissues and organs. This period is often marked by fever and other prominent and more specific signs and symptoms, which can include cough, skin lesions, diarrhea, loss of muscle control, swelling, jaundice, discharge of exudates, or severe pain, depending on the pathogen. 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 as the immune response begins to clear the infectious agent and restore normal function to dam aged tissues. In the event that the patient does not recover and dies, the infection is considered terminal.
must be considered on an individual basis. A few agents are released 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 virus, norovirus).
