Porth's Essentials of Pathophysiology, 4e

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Infection and Immunity

U N I T 4

Disease Course The course of any infectious disease can be divided into several distinguishable stages after the point when the potential pathogen enters the host. These stages are the incubation period, the prodromal stage, the acute stage, the convalescent stage, and the reso- lution stage (Fig. 14-9). The stages are based on the progression and intensity of the host’s symptoms over time. The duration of each phase and the pattern of the overall illness can be specific for different patho- gens, thereby aiding in the diagnosis of an infectious disease. The incubation period is the phase during which the pathogen begins active replication without producing recognizable symptoms in the host. The incubation period may be short, as in the case of salmonellosis (6 to 24 hours), or prolonged, such as that of hepati- tis B (50 to 180 days) or HIV (months to years). The duration of the incubation period can be influenced by additional factors, including the general health of the host, the portal of entry, and the infectious dose of the pathogen. The hallmark of the prodromal stage is the initial appearance of symptoms in the host, although the clini- cal presentation during this time may be only a vague sense of malaise. The host may experience mild fever, myalgia, headache, and fatigue. These are constitutional changes shared by a great number of disease processes. The duration of the prodromal stage can vary consider- ably from host to host. The acute stage is the period during which the host experiences the maximum impact of the infectious process corresponding to rapid proliferation and dis- semination of the pathogen. During this phase, toxic by-products of microbial metabolism, cell lysis, and the

immune response mounted by the host combine to pro- duce tissue damage and inflammation. The symptoms of the host are pronounced and more specific than in the prodromal stage, usually typifying the pathogen and sites of involvement. The convalescent period is characterized by the containment of infection, progressive elimination of the pathogen, repair of damaged tissue, and resolu- tion of associated symptoms. Similar to the incubation period, the time required for complete convalescence may be days, weeks, or months, depending on the type of pathogen and the voracity of the host’s immune response. The resolution is the total elimination of a pathogen from the body without residual signs or symptoms of disease. Several notable exceptions to the classic presenta- tion of an infectious process have been recognized. Chronic infectious diseases have a markedly protracted and sometimes irregular course. The host may experi- ence symptoms of the infectious process continuously or sporadically for months or years without a convales- cent phase. In contrast, subclinical or subacute illness progresses from infection to resolution without clini- cally apparent symptoms. A disease is called insidious if the prodromal phase is protracted; a fulminant illness is characterized by abrupt onset of symptoms with little or no prodrome. Fatal infections are variants of the typical disease course. ■■ Epidemiology is the study of factors, events, and circumstances that influence the transmission of disease. ■■ The outcomes of infections depend on the ability of microbes to breach host barriers and colonize and damage host tissues. Microbes can enter the host by direct contact, ingestion, and inhalation. The source of infection may be endogenous (acquired from the host’s own microbial flora, as would be the case in an opportunistic infection) or exogenous (acquired from sources in the external environment, such as the water, food, soil, or air). It can also be another human being, as from mother to child during gestation (congenital infections); an inanimate object; an animal; or a biting arthropod. ■■ The site of an infectious disease is determined ultimately by the type of pathogen, the portal of entry, and the competence of the host’s immunologic defense system. It may be localized to the site of entry, disseminate from the primary site of infection to involve other locations and organ systems, or travel through the circulatory system to produce disseminated infection. SUMMARY CONCEPTS

Death

Critical threshold

Chronic disease

Clinical threshold

Incubation Severity of illness and replication of pathogens Infection Prodromal

Subclinical disease

Convalescent

Acute

Resolution

FIGURE 14-9. Stages of a primary infectious disease as they appear in relation to the severity of symptoms and numbers of infectious agents.The clinical threshold corresponds with the initial expression of recognizable symptoms, whereas the critical threshold represents the peak of disease intensity.