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Inflammation and Infection 47
chronic inflammation will reveal a large number of
macrophages and fewer neutrophils.
If macrophages are unable to overcome the invader
and protect the host, the body might try to isolate the
area by forming a granuloma. A granuloma is formed by
macrophages and fibrous deposits of collagen and may
be hardened by calcium deposits. This granuloma pro
tects the surrounding tissue and allows healing to begin.
A classic cause of granuloma formation is tuberculosis.
These granulomas may become quite large, form a fibrous
rim, and eventually calcify. Another cause of granuloma is
foreign body involvement such as a wood splinter, gravel,
suture, glass sliver, or metal fragments embedded in the
tissue. The body walls off the material to protect the adja
cent tissue. This granuloma may become hardened with
fibrous tissue and remain for the life of the individual.
Inflammatory Exudates
The duration and extent of an inflammatory lesion
(LEE-zhun; any discontinuity of tissue) may be deter
mined by direct visualization of the site. External
inflammatory lesions are observed easily, whereas
internal inflammatory lesions in organs and cavities
might require surgical or endoscopic examination. The
appearance and amount of exudate or blood fluid can
assist in identifying an acute or a chronic condition.
Serous Exudate
Serous exudate is a clear, serum-like fluid containing
small amounts of protein. It implies a lesser degree of
damage and occurs in the acute stage of inflammation.
Examples of serous exudate include the fluid in skin
blisters, cold sores, and injured joints, for example.
Serous exudate is easily reabsorbed after the inflamma
tory response is halted and healing begins.
Fibrinous Exudate
Fibrinous exudate is composed of fluid and large
amounts of fibrinogen. In comparison to serous exu
date, the leakage of fibrinogen indicates a larger injury
with more severe inflammation. Fibrinous exudate can
be observed in strep throat or bacterial pneumonia,
forming a mesh-like lesion. A superficial skin wound
might be covered with dried fibrinous exudate com
monly called a scab.
Healthy Highlight
Emerging Infectious
Diseases: How to
Stay Healthy
T
here are several diseases on the rise in the United States today, such as Escherichia
coli (E. coli), tuberculosis, Shigella, and others, but there are also some “new emerg
ing” infectious diseases on the horizon. Since the advent of antibiotics, there have been
more deaths from chronic diseases in this country than from acute infectious diseases.
However, this could change in the future as new treatments for chronic diseases are
discovered, and, since diseases not previously seen in humans or in this country are
now being seen such as COVID-19. Some other emerging diseases include hantavirus
pulmonary syndrome (HPS), West Nile Virus, monkeypox disease, bovine spongiform
encephalopathy (BSE)/variant Creutzfeldt-Jakob Disease (vCJD), Hansen’s disease,
smallpox, and rabies. A few of these were seen here in the past but were thought to be
eliminated. There are many more infectious diseases, such as Zika, Hendra, and Nipah
virus diseases, that have existed in other countries but are now spreading to the United
States. A few, like Lyme disease, community-acquired methicillin-resistant Staphylo
coccus aureus (CA-MRSA), group A streptococcal (GAS) disease, avian influenza, and
meningococcal disease, were once also unknown or very uncommon but have now
become almost commonplace. This has been a concern at the Centers for Disease Con
trol and Prevention (CDC) and will continue to be studied. In the future, there might
be new vaccines for some of these diseases that presently have no vaccines available.
COVID-19 is one disease that because of its pandemic spread, developing a vaccine
was a priority. Using proper handwashing, avoiding ill individuals, and, in some cases,
wearing a mask and using sanitizing products are important tips for staying healthy.
48 Chapter 4
Purulent Exudate
Purulent (PURR-you-lent) exudate is loaded with dead
and dying PMNs or neutrophils, tissue debris, and
pyogenic (PYE-oh-JEN-ick; pyo 5 pus, genic 5 aris
ing), or pus-forming, bacteria. Purulent exudate is com
monly called pus. A localized collection of pus is called
an abscess; an accumulation of pus in a body cavity is
called empyema (EM-pye-EE-mah). For example, pus
accumulated in the chest or thoracic cavity would be
called thoracic empyema.
Inflammatory Lesions
Any discontinuity or abnormality of tissue is called
a lesion, a broad term that includes wounds, ulcers,
wheals, blisters, vesicles, pustules, or tumors, to name
a few. Lesions are due to physical or pathologic injury.
Inflammatory lesions include abscesses, ulcers, and cel
lulitis (SELL-you-LYE-tis; inflammation of connective
tissue).
Abscesses
Abscesses are typically caused by streptococcal and
staphylococcal (pyogenic) bacteria. During the
inflammatory response, the body attempts to con
tain or stop the spread of the bacteria into adjacent
tissue by forming a wall around the area. When this
wall forms around a purulent exudate, an abscess is
formed. Boils, furuncles, and pimples are examples
of abscesses.
Typically, a small abscess shows signs of acute inflam
mation: redness, heat, swelling, and pain. When the
central portion of the abscess softens or develops a head,
puncturing the head will cause an outpouring of pus, the
relief of pain, and the onset of healing. Puncturing the
abscess before the area is walled off and the head is soft,
however, can lead to a spread of the infecting organism.
A small abscess might rupture and heal sponta
neously, but a large abscess might need to be surgi
cally incised and drained. Draining an abscess speeds
healing; without drainage, the body must continue to
battle the invading organisms. If the body is successful,
it will eventually win the battle, reabsorb the exudate,
and replace the area with fibrous tissue. A large abscess,
such as in appendicitis, can spread and become fatal if
not contained. If a large abscess ruptures, it tends to
form a tract, or opening to the surface of the skin called
a sinus. If this tract connects two organs or cavities to
each other or to the surface of the skin, it is called a
fistula (FIST-you-lah; Figure 4-3).
Ulcer
An ulcer is a crater-like lesion in the skin or mucous
membrane. It is the result of an injury and the subse
quent inflammatory response. The tissue in this area
becomes necrotic (dead) and sloughs off, leaving a cra
ter or excavated area. Ulcers are commonly seen in the
stomach and duodenum as a result of injury by bacte
ria and stomach acid. Pressure ulcers, commonly called
bedsores or decubitus ulcers, are caused by excessive
pressure on tissue. Pressure ulcers primarily appear
over bony prominences of the body, especially those
affected in the reclining position, such as the heel,
sacrum, hip, elbow, and scapula (Figure 4-4).
(B)
Cystovaginal
fistula allows
urine to run
out the vagina
Figure 4-3 (A) Sinus. (B) Fistula.
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Inflammation and Infection 49
Figure 4-4 Pressure ulcer.
Cellulitis
Cellulitis is a diffuse, or widespread, acute inflam
matory process. It is commonly seen in the skin and
subcutaneous tissues. Cellulitis is characterized by gen
eral edema and redness. Cellulitis of the face primar
ily involves the cheeks and periorbital (peri 5 around,
orbital 5 eye) areas. This type of cellulitis must receive
special attention because it can spread to the sinuses of
the brain. Cellulitis is often caused by Streptococcus or
Staphylococcus bacteria and is due to the body’s inabil
ity to confine or wall off the causative organism. Cellu
litis is potentially dangerous but usually can be treated
effectively with antibiotics.
Tissue Repair and Healing
Proper tissue repair and healing is an ongoing process
much like any other body process and occurs in most
instances, but this process can be influenced by many
other factors. Healing can be impaired or slowed when
secondary diseases are present, the body is malnour
ished, or the immune system is compromised.
Tissue Repair
During the final phase of the inflammatory process,
macrophages are responsible for cleaning up the area
and producing growth factors that aid the repair pro
cess. Repair of tissue also depends on cellular regenera
tion and the type of cells that make up the tissue. Some
cells divide quite readily, but others do not. Cellular
proliferation, or division, can be grouped into three
general categories.
1. Mitotic cells continuously divide throughout
life. They exist in the skin and mucosa of inter
nal organs and readily replace damaged tissue.
2. Facultative mitotic cells do not divide regularly
but can be stimulated to divide when necessary.
They exist in such organs as the liver and kid
ney. Some part of these organs must remain
intact for these cells to be available to divide and
replace the lost tissue.
3. Non-dividing cells do not divide under any
condition. Cells of this type include central
nerves, brain cells, and heart muscle cells.
Repair of these tissues is by fibrous scarring.
The body’s two basic methods of repair involve
healing by regeneration and by fibrous connective tis
sue repair, or scar formation. Regeneration is the best
type of repair because it usually leads to the restoration
of normal function, whereas fibrous connective tissue
repair does not.
Regeneration
Regeneration involves mitotic cell division. During
regeneration, the damaged tissue is replaced by the
cellular division of healthy tissue. For example, skin
tissue is replaced by epithelial cell division, and bone
tissue is replaced by osteocyte division. Regenera
tion can usually occur in internal organs if the major
framework of the organ has not been destroyed. Com
plex structures such as lung tissue and glomeruli (in
the kidney), however, do not regenerate. Regeneration
is particularly important when there is damage to a
large amount of tissue; for instance, epithelial regen
eration is very beneficial with massive burns. Bone
cells, osteocytes, and blood-forming bone cells have a
remarkable ability to regenerate from a few remaining
cells and can be transplanted from another individual
by bone marrow transplant.
Fibrous Connective Tissue
Repair (Scar Formation)
Fibrous connective tissue repair, or scar formation, can
occur in any tissue and produces the same result, no
matter the location—a tough, fibrous tissue called a
scar. A scar provides a bridge between the normal tissue
and the wound, but it does not restore function. Wound
repair of nerves, brain tissue, and heart muscle is by
fibrous connective tissue repair (Figure 4-5).
50 Chapter 4
Courtesy of Mark L. Kuss
Figure 4-5 Tissue repair—complete regeneration and fibrous connective tissue repair.
Tissue Healing
Tissue healing can be separated into categories of heal
ing by primary union or secondary union. Categori
zation is determined by whether the wound edges are
approximated (pulled together) or left separated during
the healing process.
Primary Union (First Intention)
Primary union, also called healing by first intention,
involves approximating the edges of the wound. A
classic example of healing by primary union is the
healing process following a clean surgical incision.
The wound edges are clean, there is minimal tissue
damage, and the edges are approximated with sutures,
staples, or tape.
Primary healing occurs in an orderly fashion. The
steps of primary healing include the following:
1. The incisional line quickly fills with serum,
forming a scab.
2. Within 1 to 2 days, new capillaries begin to
bridge the gap between the wound edges.
3. In the next few days, fibroblasts grow across the
deeper wound layers and begin to deposit colla
gen in this fibrous network. This tissue is called
granulation tissue.
4. The collagen begins to contract, pulling the
wound edges together and forming a scar.
After a few weeks, the incision might appear healed,
but the deeper layers of tissue might not be healed for a
month or more. Usually, an incisional scar will pale in
color and shrink in size over a period of months or years.
Secondary Union (Secondary Intention)
Large wounds and those infected by dirt, debris, and bac
teria cannot be pulled together to heal by primary inten
tion. The process of healing by secondary union is the
same process as that of primary union but involves a larger
degree of tissue damage and more inflammation to resolve
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Inflammation and Infection 51
Small amount of exudate
Large amount of exudate Large amount of granulation tissue
Secondary union
Small-sized scar
Large-sized scar
Figure 4-6 Tissue healing — primary and secondary union.
(Figure 4-6). To fill the wound, large numbers of capil
laries, fibroblasts, and collagen must be produced. After
a week or so, the new, soft, red tissue is called granula
tion tissue, which is eventually replaced as more collagen
is deposited in the area. The collagen contracts, pulling
the wound edges together and beginning the formation
of a scar. Healing time varies depending on the size of the
wound; large wounds can take a long time to heal by sec
ondary union because additional time will be needed for
the scar to develop the strength of the surrounding tissue.
If the wound is too large, the epithelium might not be able
to bridge the gap, and a skin graft might be needed.
Delayed Wound Healing
One of the greatest impediments to wound healing is
the amount of dead tissue and debris—dirt, bacteria,
dead leukocytes, or a variety of other contaminants—in
the wound. It might take the body’s leukocytes weeks
or months to phagocytize (eat up) all the debris. In the
meantime, bacteria might be producing dead cells and
necrotic tissue as fast as the cleanup effort can advance.
To speed healing, dirty wounds must be cleaned and
debrided. Debridement (day-breed-MON), commonly
pronounced (day-breed-MENT), is a process of washing
or cutting away necrotic tissue and foreign material.
Consider This...
Maggot treatment, also called biotherapy,
is often more effective than modern antibiot
ics for treating open wounds. Maggots will
gently eat away the decaying tissue, will
leave the healthy tissue intact, and will not
cause any side effects.
Other factors affecting healing time include the
following:
1. Age Younger people heal more rapidly than
older people.
2. Size Smaller wounds heal faster than larger
ones.
3. Location Epithelial tissue heals rapidly, com
pared to other tissue types.
4. Nutrition Good nutritional status promotes
wound healing. Protein and vitamin C are
essential to healing.
5. Immobility Wound tissue heals more rapidly if
it is kept immobile.
6. Circulation Tissue with good blood supply
heals more rapidly. Epithelial tissue heals more
readily than cartilage. Individuals with diabe
tes have small-blood-vessel disease (diabetic
micro-angiopathy), leading to ischemia of the
tissue and poor wound healing.
7. Organism virulence Wounds infected with
virulent (VIR-you-lent; poisonous) micro
organisms are slower to heal than those not
infected.
8. Steroids Steroid therapy inhibits the inflamma
tory response, giving the invading offender the
upper hand.
Complications of Wound Healing
Prolonged wound healing can occur as a result of any one
or a combination of the factors previously discussed. Other
complications of wound healing involve poor or exces
sive scar formation. A scar that does not have adequate
strength can lead to wound dehiscence (dee-HISS-ens),
52 Chapter 4
Figure 4-7 Keloid.
or separation of tissue margins. Excessive collagen for
mation often results in a hard, raised scar called a keloid
(KEE-loid; Figure 4-7). Keloid scars are often unsightly
but harmless and occur more often in Blacks. Surgical
removal can result in the formation of another keloid.
One potential complication of surgery, especially
abdominal surgery, is adhesion (ad-HE-zhun) from
scar tissue. As normal fibrous scar tissue develops in
the operative organ, part of this tissue can cling to the
surface of the adjoining organs, forming fibrous bands
called adhesions. Adhesions are often asymptomatic
and cause no difficulties, but in some cases, they can
become painful and lead to obstruction of the adjacent
organ. The intestine frequently is obstructed by adhe
sions following abdominal surgery. Further surgery to
release painful or obstructive adhesions may be needed.
Infection
In Chapter 2, “Mechanisms of Disease,” the fact that
inflammation and infection are different responses or
reactions was introduced. Inflammation is a protec
tive immune response and can occur without bacterial
invasion. Infection, however, refers to the invasion
of microorganisms in the tissue, causing cell or tissue
injury and leading to the inflammatory response.
Humans live with disease-causing microorganisms
all around them. Some bacteria even live on the skin
surface, in the respiratory tract, and in the intestine
without causing illness, and some are actually benefi
cial. These bacteria are called normal flora.
Microorganisms that produce disease are called
pathogenic. Normal flora can become pathogenic under
certain conditions. When this occurs, the normal flora
bacteria become opportunistic because they take the
opportunity to cause infection in the host.
Certain conditions must be present for a micro
organism to cause an infection in the host. Pathogens
must have an area to enter, be resistant enough, and
enter in great enough numbers to survive, and over
come the defenses of the individual.
First, the microorganism must successfully gain
access into the body through a portal of entry; any break
in the skin offers this. Common openings such as the
nose, mouth, eyes, and ears are also portals of entry.
The most common port of entry is the respiratory sys
tem. Other portals include the digestive system, urinary
tract, and reproductive tract.
Second, the pathogen must be resistant to the
defenses of the host. The ability of a microorganism
to overcome the defense of the host is its virulence. A
virulent microorganism has an aggressive or invasive
nature and can produce a toxin, or poison, that injures
tissues. The degree of virulence of a microorganism var
ies. Generally, organisms that come from an infected
host are more virulent than those grown in laboratory
conditions.
Third, the number of invading pathogens can
determine the risk for infection—even weak patho
genic organisms can cause infection if they invade in
large enough numbers to overcome the body’s defense
system. Generally, the higher the number of invading
pathogens, the greater the risk of infection.
Finally, the condition of the individual or the host is
a determinant of infection risk. An individual who is in
good physical and emotional health, has good nutrition,
practices risk-reducing habits, and is relatively young
has a good chance of avoiding infection.
Consider This...
'1
The average office desk has 400 times
more bacteria than a toilet seat!
Frequency and Types of Infection
Infectious diseases are the leading cause of death in the
world (Table 4-1), so a country’s ability to track and
identify infectious diseases is an important weapon in
the control of disease. In the United States, the CDC,
based in Atlanta, provides these services.
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Inflammation and Infection 53
TABLE 4-1 Some of the Leading Causes of Death in the
World Due to Infections
■ Respiratory Infections
■ HIV/AIDS
■ Diarrheal Disease
■ Tuberculosis
■ Malaria
■ Measles
■ Pertussis
■ Tetanus
Respiratory infections, including upper respira
tory infections, influenza-like infections, pneumonia,
and bronchitis, account for more than 80% of all infec
tions. Childhood infections, wound infections, viral
infections, and other types of infection account for the
remaining number of infections diagnosed.
Microorganisms that produce infection in humans
include bacteria, viruses, fungi, rickettsiae (ric-KET-
see-ah), protozoa, and helminths (Table 4-2). These
organisms can produce infections in the host that range
from very mild to life-threatening.
Bacteria
Bacterial infections (Figure 4-8) can occur as a pri
mary or secondary disease. Primary bacterial infections
Figure 4-8 Bacteria.
occur when a person is exposed to a pathogen. Sec
ondary infection occurs after the onset of another dis
ease process or condition. Secondary infections are
very common. The most common cause for them is
obstruction of a body passageway. For example, a nasal
obstruction can lead to sinusitis, and obstruction of the
TABLE 4-2 Some Common Infections Caused by Microorganisms in Humans
Bacteria
Virus Fungus
Staphylococcus Common cold Ringworm (tinea)
Streptococcus Herpes simplex Athlete’s foot
Escherichia coli Mononucleosis Candidiasis
Klebsiella HIV Thrush
Pseudomonas Measles Vaginitis
Shigella Mumps Histoplasmosis
Salmonella Rubella Co ccidioidomycosis
Influenza (flu)
Rickettsial Protozoan
Helminths
Rocky Mountain Malaria Roundworms
spotted fever Giardiasis Flatworms
Pinworms
Tapeworms
54 Chapter 4
eustachian tubes can lead to otitis media, or middle-ear
infection.
Normal flora bacteria live on or in the skin, mouth,
nose, genital tract, and intestines of humans. These bac
teria often become pathogenic when they gain access
into the body or when the body’s resistance is less
robust than normal. Staphylococcus is a bacterium of
the skin that often enters the body and can infect any
organ. Staphylococcus aureus is an important member
of the Staphylococcus family because it can develop
strains, such as methicillin-resistant Staphylococcus
aureus (MRSA), that are resistant to penicillin and
other antibiotics. These antibiotic-resistant strains are
particularly dangerous because they are difficult to
control and eliminate. For instance, a form of MRSA
called CA-MRSA is the most dangerous form of MRSA.
It has become epidemic in the United States in the past
few years. It is estimated that 2 in every 100 people
now carry the infection without symptoms. It is sec
ond only to human immunodeficiency virus (HIV) as
the major public health threat. CA-MRSA starts out as
a skin infection but soon becomes a serious systemic
infection. It can rapidly cause severe respiratory distress
leading to respiratory failure.
Consider This...
Every square inch of skin has about 32 mil
lion bacteria living on it.
Streptococcus bacteria normally live on the skin
and in the throat. Common infections caused by Strep
tococcus bacteria include strep throat, scarlet fever,
pneumonia, and meningitis. In a select group of indi
viduals, strep throat can lead to rheumatic fever and
glomerulonephritis.
Consider This...
There are more bacteria in your mouth than
there are people in the world!
Enteric bacteria are those living in the intestinal
tract, and common forms include E. coli, Klebsiella,
Pseudomonas, Shigella, and Salmonella. E. coli causes
enteritis in infants and adults and can be the cause of
travelers’ diarrhea. E. coli and Klebsiella are common
causes of urinary tract infections. Pseudomonas com
monly infects wounds and is associated with a foul
odor and green pus production. Shigella and Salmonella
infections cause diarrhea; Salmonella is the causative
organism of food poisoning.
Viruses
Viruses (Figure 4-9) are the smallest infective organ
isms and must be visualized by an electron micro
scope. They cannot reproduce or live outside the cell
and must invade the cell and use it to reproduce their
Complementary and Alternative Therapy
Herbs for Infections variety of herbs have historically been used to treat infections, and many of these
are still widely used today. Sepsis is a life-threatening complication of some infec
tions. There are many treatment plans to deal with sepsis in an individual but seldom
are herbal plants used for this condition in the United States. A variety of herbal plants
found in Malaysia have been used as a treatment for sepsis. Ginger is one that has shown
potential for treating sepsis. It may be used in several forms such as dried or an extract.
Curcumin has also shown some positive effects when used to treat sepsis. The amount
needed of the herb to be an effective treatment has not been established. Caution is also
recommended because not all these treatments have been fully researched and proved.
Source: Liew (2020)
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Inflammation and Infection 55
Figure 4-9 Virus.
genetic information. Lymphocytes of the immune
system are the body’s primary defense against viruses.
Some viruses can mutate or change, so the body must
develop more than one kind of antibody to kill that
type of virus.
Viral infections cannot be treated easily. Some
antiviral agents can be given to individuals who have
reduced resistance to infections to try to prevent such
an infection. Antibiotic therapy does not kill a virus.
Usually, supportive care is given by treating the symp
toms the virus causes, such as fever, sore throat, runny
nose, headache, and chest congestion. Antibiotics will
help in treating a secondary bacterial infection occur
ring with the viral infection.
Viral infections of the upper respiratory system,
including the common cold, far outnumber other viral
diseases. Cold sores, also known as herpes simplex,
are very common and affect many individuals. Infec
tious mononucleosis frequently affects adolescents and
young adults. HIV causes acquired immunodeficiency
syndrome (AIDS) and has become the most noted virus
due to its fatal outcome.
Immunizations are effective in preventing many
viral diseases such as measles, mumps, rubella, and
smallpox. Influenza virus (flu) mutates and requires
new vaccines with each mutation. Some viruses are
latent types, meaning they live inside the cell, causing
no harm until the body becomes stressed or impaired.
Latent viruses, such as those in the herpes family, repli
cate and cause symptoms during stressful periods.
Fungi
Fungi (Figure 4-10) are microscopic plantlike organ
isms that cause diseases referred to as mycoses. Fungi
Figure 4-10 Fungi.
are larger than bacteria, and only a few types are
pathogenic. Single-celled forms of fungi are called
yeast.
Fungal infections of the skin such as those of the
tinea family (ringworm and athlete’s foot) are com
mon. Candida, commonly called candidiasis or yeast
infection, often occurs in individuals with suppressed
immune systems, anyone on long-term antibiotic
therapy, and in individuals with diabetes. Candida is
a superficial infection of the skin and mucous mem
branes, appearing commonly in the moist folds of the
skin, the mouth (thrush), vaginal cavity (vaginitis), and
genital area.
Other fungal infections include histoplasmo
sis and coccidioidomycosis (cok-sid-e-oyd-o-my-
CO-wsis). These infections are common to certain
geographical locations but are not common in the
general population. Fungal infections can be treated
with antifungal and antibiotic medications but often
are difficult to cure and might require long-term
therapy.
Rickettsiae
Rickettsiae (Figure 4-11) are microscopic organisms
that are intermediate between bacteria and viruses.
They must live in the host cell like a virus. Rickettsiae
are spread by fleas, ticks, mites, and lice and can cause
fatal infections in humans, the most common of which
is Rocky Mountain spotted fever.
56 Chapter 4
Healthy Highlight
Medication
Precautions
W
ARNING! Anyone taking a prescribed antibiotic medication should always take
ALL the medication even if the symptoms stop. Antibiotics should not be saved
for the next illness. Failing to complete antibiotic therapy can lead to the development
of antibiotic-resistant strains of bacteria because the first doses of medication might
kill weaker bacteria and stun the stronger ones, but if therapy is halted prematurely,
the stronger bacteria might survive and reproduce strains that can resist the antibiotic.
When this occurs, stronger and usually more expensive medications must be used to
treat the same infection at a later date. Mismanagement of antibiotic therapy has led to
the development of strains of bacteria that now must be treated with stronger oral anti
biotics or intravenous antibiotics.
Healthy Highlight
Prevention for the
Common Cold
T
he common cold virus has more than 100 strains, and the body must identify and
kill each virus as the body becomes infected. An individual will not suffer with the
same cold virus twice, which explains why young children have more colds than adults.
As we age, we have become ill with many cold viruses and have developed immunity
to this greater number. Cold viruses are very contagious, entering the body primarily
through the respiratory tract. Good handwashing is the best preventive measure for the
common cold, and using antibacterial hand gels and sanitizers is also recommended.
Protozoa
Protozoa (Figure 4-12) are single-celled microscopic
members of the animal kingdom. They are found in
the soil and live on dead or decaying material. Infection
is by ingesting spores or by infected insect bites.
Figure 4-12 Protozoa.
Courtesy of Mark L. Kuss
Figure 4-11 Rickettsiae.
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