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Infectious Diseases

Author(s): Christine Herrmann, PhD

Phases of Infectious Disease

Diseases caused by infectious agents usually run a standard course that is associated with different signs and symptoms. Before we describe these different phases of infectious diseases, we must first define the terms, “signs” and “symptoms.” Signs of an infectious disease are characteristics of a disease that can be observed by examining a patient. They include things such as fever, coughing, rash, vomiting, and diarrhea. Symptoms, on the other hand, can be felt only by the patient. They include pain, headache, and nausea.

The time period from when a person first becomes infected until signs and symptoms become apparent is called the incubation period. During this phase, the person is not aware that he or she is infected, but he or she may already be contagious (capable of passing the agent on to others). The length of the incubation period is typical for a specific agent but can vary, depending on how virulent the agent is, the dose of infectious agent entering the body, and the route of infection (i.e., where the agent enters the body relative to the tissue it infects).

In the prodromal phase, a person experiences mild, nonspecific symptoms. During this time, the agent is continuing to multiply and the person is contagious. This phase is absent in some diseases, which cause a person to feel ill suddenly, without any warning.

The clinical phase (also called the invasive phase or acute phase) is the period in which typical disease signs and symptoms are evident. During this phase, there comes a time when symptoms reach their greatest intensity. Called the “acme,” this is the height of the battle between the pathogen that is invading and destroying tissue and the efforts of the body’s immune system to contain and obliterate the invader. Fever is usually a component of this phase, during which the patient is most contagious. Once the acme is reached, the number of infectious agents begins to drop and the signs and symptoms start to decrease. This is the decline phase, during which the body’s activities gradually return to normal, the tissues heal, and the individual no longer experiences any symptoms. The recovery phase also is known as the convalescent phase. With some diseases, such as chickenpox, a person can still be contagious during recovery until the lesions are healed. 

From the pathogen’s perspective, it is advantageous to induce the signs and symptoms of disease, such as coughing, sneezing, vomiting, and diarrhea, because these are ways that the infectious agent can be released from one host and spread to another. And although we do not enjoy feeling ill, these signs and symptoms are beneficial to us, in that they make us more likely to stay in bed and conserve energy to fight the pathogen and minimize the chance of spreading the disease to others.