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U N I T 4
Infection and Immunity
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.
Incubation
Prodromal
Acute
Convalescent
Subclinical disease
Chronic disease
Critical threshold
Clinical
threshold
Death
Severity of illness and
replication of pathogens
Infection
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.
SUMMARY CONCEPTS
■■
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.
