generalized muscle aches, lethargy, and fatigue, or a
feeling that “I’m coming down with something.”
During the invasion period, the specific symptoms
of the illness appear. These might include a high fever,
rash, swollen lymph nodes, cough, diarrhea, or such
things as the gradual paralysis of botulism. The acme
is the worst stage of the disease, and this is followed by
the recovery or death of the host.
Some diseases are self-limiting; that is, they typi-
cally last a certain length of time and are usually fol-
lowed by recovery. The common cold, chickenpox, and
mumps are illnesses that are considered self-limiting.
TYPES OF INFECTION
The terminology of infection may refer to the location
of the pathogens in the body, to the general nature of
the disease, or to how or where the pathogen was
acquired.
A localizedinfection is one that is confined to one
area of the body. Examples are the common cold of
the upper respiratory tract, boils of the skin, and sal-
monella food poisoning that affects the intestines.
In a systemicinfection, the pathogen is spread
throughout the body by way of the lymph or blood.
Typhoid fever, for example, begins as an intestinal
infection, but the bacteria eventually spread to the
liver, gallbladder, kidneys, and other organs. Bubonic
plague is an infection that begins in lymph nodes, but
again the bacteria are carried throughout the body,
and fatal plague is the result of pneumonia.
Bacteremiaand septicemia are terms that are
often used interchangeably in clinical practice,
although technically they have different meanings.
Bacteremia means that bacteria are present in the
blood; septicemia means that the bacteria present are
multiplying rapidly. Septicemia is always serious,
because it means that the immune defenses have been
completely overwhelmed and are unable to stop the
spread of the pathogen.
With respect to timing and duration, some infec-
tions may be called acute or chronic. An acuteinfec-
tion is one that usually begins abruptly and is severe.
In contrast, a chronic infection often progresses
slowly and may last for a long time.
A secondary infectionis one that is made possible
by a primary infection that has lowered the host’s
resistance. Influenza in an elderly person, for example,
may be followed by bacterial pneumonia. This sec-
ondary bacterial infection might not have occurred
had not the person first been ill with the flu.
Nosocomialinfections are those that are acquired
in hospitals or other institutions such as nursing
homes. The hospital population includes newborns,
the elderly, postoperative patients, people with serious
chronic diseases, cancer patients receiving chemother-
apy, and others whose resistance to disease is lowered.
Some hospital-acquired pathogens, such as Staphylo-
coccus aureus, are transmitted from patient to patient by
healthy hospital personnel. These staff members are
reservoirs for S. aureusand carry it on their skin or in
their upper respiratory tracts. For this reason, proper
hand washing is of critical importance for all hospital
staff.
Other nosocomial infections, however, are caused
by the patient’s own normal flora that has been inad-
vertently introduced into an abnormal body site. Such
infections may be called endogenous, which literally
means “generated from within.” Intestinal bacilli such
as E. coliare also an important cause of nosocomial
infections. Without very careful aseptic technique
(and sometimes in spite of it), the patient’s own intes-
tinal bacteria may contaminate urinary catheters,
decubitus ulcers, surgical incisions, chest tubes, and502 An Introduction to Microbiology and Human Disease
Table 22–2 INCUBATION PERIODS OF
SOME INFECTIOUS DISEASESDisease Incubation Period
Chickenpox 14–16 days
Cholera 1–3 days
Diphtheria 2–6 days
Gas gangrene 1–5 days
Gonorrhea 3–5 days
Hepatitis A 2 weeks–2 months
Hepatitis B 6 weeks–6 months
Hepatitis C 2 weeks–6 months
Herpes simplex 4 days
Influenza 1–3 days
Leprosy 3 months–20years
Measles 10–12 days
Meningitis (bacterial) 1–7 days
Mumps 2–3 weeks
Pertussis 5 days–3 weeks
Pinworm 2–6 weeks
Plague 2–6 days
Polio 7–14 days
Rabies 2 weeks–2 months
(up to 1 year)
Salmonellafood poisoning 12–72 hours
Staphylococcusfood poisoning 1–8 hours
Syphilis 10 days–3 months
Tetanus 3 days–5 weeks
Tuberculosis 2–10 weeks