Despite improved methods of treatment and prevention — potent antibiotics, complex immunizations, and modern sanitation — infection still accounts for much serious illness, even in highly industrialized countries. In developing countries, infection is one of the most critical health problems.
Infection is the invasion and multiplication of microorganisms in or on body tissue that produce signs and symptoms as well as an immune response. Such reproduction injures the host by causing cellular damage from microorganism-produced toxins or intracellular multiplication or by competing with host metabolism. The host's own immune response may compound the tissue damage, which may be localized (as in infected pressure ulcers) or systemic. The severity of the infection varies with the pathogenicity and number of the invading microorganisms and the strength of host defenses. The very young and the very old are especially susceptible to infections.
Why are the microorganisms that cause infectious diseases so difficult to overcome? There are many complex reasons:
❑Some bacteria develop a resistance to antibiotics.
❑Some microorganisms — such as human immunodeficiency virus — include so many different strains that a single vaccine can’t provide protection against them all.
❑Most viruses resist antiviral drugs.
❑Some microorganisms localize in areas that make treatment difficult, such as the central nervous system and bone.
Moreover, certain factors that contribute to improved health — such as the affluence that allows good nutrition and living conditions and advances in medical science — can increase the risk of infection. For example, travel can expose people to diseases for which they have little natural immunity. The expanded use of immunosuppressants, surgery, and other invasive procedures also increases the risk of infection.
A laboratory-verified infection that causes no signs and symptoms is called a subclinical, silent, or asymptomatic infection. A multiplication of microbes that produces no signs, symptoms, or immune response is called a colonization. A person with a subclinical infection or colonization may be a carrier and transmit the infection to others. A latent infection occurs after a microorganism has been dormant in the host, sometimes for years. An exogenous infection results from environmental pathogens or sources other than the host; an endogenous infection, from the host’s normal flora (for instance, Escherichia coli displaced from the colon, which causes urinary tract infection).
The varied forms of microorganisms responsible for infectious diseases include bacteria, viruses, rickettsiae, chlamydiae, fungi (yeasts and molds), and protozoa; larger organisms such as helminths (parasitic worms) may also cause infectious disease.
Bacteria are single-cell microorganisms with well-defined cell walls that can grow independently on artificial media without the need for other cells. Bacteria inhabit the intestines of humans and other animals as normal flora used in the digestion of food. Also found in soil, bacteria are vital to soil fertility. These microorganisms break down dead tissue, which allows it to then be used by other organisms.
Despite the numerous types of known bacteria, only a small percentage are harmful to man. (See How bacteria damage tissue, page 154.) In developing countries, where poor sanitation increases the risk of infection, bacterial diseases commonly result in death and disability. In industrialized countries, bacterial infections are the most common fatal infectious diseases.
Bacteria can be classified according to shape. Spherical bacterial cells are called cocci; rod-shaped bacteria, bacilli; and spiral-shaped bacteria, spirilla. Bacteria can also be classified according to their response to staining (gram-positive, gram-negative, or acid-fast bacteria); their motility (motile or nonmotile bacteria); their tendency toward encapsulation (encapsulated or nonencapsulated bacteria); and their capacity to form spores (sporulating or nonsporulating bacteria).
Spirochetes are bacteria with flexible, slender, undulating spiral rods that have cell walls. Most are anaerobic. The three forms pathogenic in humans include Treponema, Leptospira, and Borrelia.
Viruses are subcellular organisms made up only of a ribonucleic acid or a deoxyribonucleic acid nucleus covered with proteins. they're the smallest known organisms (so tiny they're visible only through an electron microscope). Independent of host cells, viruses can't replicate. Rather, they invade a host cell and stimulate it to participate in the formation of additional virus particles. The estimated 400 viruses that infect humans are classified according to their size; shape (spherical, rod shaped, or cubic); or means of transmission (respiratory, fecal, oral, or sexual).
Rickettsiae are relatively uncommon in the United States. they're small, gram-negative organisms classified as bacteria that commonly induce life-threatening infections. Like viruses, they require a host cell for replication. Three genera of rickettsiae include Rickettsia, Coxiella, and Rochalimaea.
Chlamydiae are smaller than rickettsiae and bacteria but larger than viruses. They too depend on host cells for replication but, unlike viruses, they're susceptible to antibiotics.
Fungi are single-cell organisms, with nuclei enveloped by nuclear membranes. They have rigid cell walls like plant cells but lack chlorophyll, the green matter necessary for photosynthesis; they also show relatively little cellular specialization. Fungi occur as yeasts (single-cell, oval-shaped organisms) or molds (organisms with hyphae, or branching filaments). Depending on the environment, some fungi may occur in both forms. Fungal diseases in humans are called mycoses.
Protozoa are the simplest single-cell organisms of the animal kingdom. However, they show a high level of cellular specialization. Like other animal cells, they have cell membranes rather than cell walls, and their nuclei are surrounded by nuclear membranes.
In addition to these microorganisms, infectious diseases may also result from larger parasites, such as roundworms or flatworms.
Most infectious diseases are transmitted in one of four ways (see Standard precautions; also see CDC isolation precautions, page 157):
❑In contact transmission, the susceptible host comes into direct contact (as in contact with blood or body fluids) or indirect contact (contaminated inanimate objects or the close-range spread of respiratory droplets) with the source. The most common method of contact transmission is contaminated hands.
❑Airborne transmission results from the inhalation of contaminated aerosolized droplet nuclei (as in pulmonary tuberculosis).
❑In enteric (oral-fecal) transmission, the infecting organisms are found in feces and are ingested by susceptible victims, in many cases through fecally contaminated food or water (as in salmonella infections).
❑Vector-borne transmission occurs when an intermediate carrier (vector), such as a flea, mosquito, or other animal, transfers an organism.
Much can be done to prevent the transmission of infectious diseases:
❑comprehensive immunization (including the required immunization of travelers to, or emigrants from, endemic areas)
❑drug prophylaxis
❑improved nutrition, living conditions, and sanitation
❑correction of environmental factors
❑widespread disease tracking.
Immunization can now control many diseases, including diphtheria, tetanus, pertussis, measles, rubella, some forms of meningitis, poliovirus, hepatitis B, pneumococcal pneumonia, influenza, rabies, and tetanus. Smallpox (variola) — which killed and disfigured millions — was believed to have been successfully eradicated by a comprehensive World Health Organization program of surveillance and immunization. However, in light of recent concerns regarding bioterrorism, smallpox is considered a potential agent. Health care personnel must recognize potential cases of smallpox and initiate appropriate precautions as well as notify health department officials. Smallpox vaccination may be appropriate for certain emergency and first-response health care providers.
Vaccines — which contain live but attenuated (weakened) or killed microorganisms — and toxoids — which contain modified bacterial exotoxins — induce active immunity against bacterial and viral diseases by stimulating antibody formation. Natural active immunity is produced as a patient who has the disease forms antibodies against it, thus preventing the recurrence of the disease. Immune globulins contain previously formed antibodies from hyperimmunized donors or pooled plasma and provide temporary passive immunity. Generally, passive immunization is used when active immunization is perilous or impossible or when complete protection requires both active and passive immunization. It may also be appropriate in situations requiring immediate protection such as postexposure in which active immunity from immunizations takes too long to provide the necessary — and immediate — protection. Maternal passive immunity crosses the placental barrier from mother to fetus and is also provided to the infant by antibodies present in breast milk.
Although prophylactic antibiotic therapy may prevent certain diseases, the risk of superinfection and the emergence of drug-resistant strains may outweigh the benefits. Therefore, prophylactic antibiotics are usually reserved for patients at high risk for exposure to dangerous infections. Antibiotic-resistant bacteria are on the rise mainly because antibiotics have been misused and overused. Some bacteria, such as enterococci, have developed mutant strains that don't respond to antibiotic therapy.
A health care-associated infection is an infection that develops as a result of health care. Health care-associated infections were previously known as nosocomial infections, but the name was updated because these infections may be acquired from, or associated with, any portion of the health care-delivery system, including such areas as outpatient care, ambulatory care, home care, or long-term care.
Health care-associated infections are usually transmitted by direct contact. Less commonly, transmission occurs by inhalation or by contact with contaminated equipment and solutions. Contamination of solutions during the manufacturing process is rare.
Despite facility programs of infection control that include surveillance, prevention, and education, about 5% of patients who enter health care facilities contract a health care-associated infection. Staphylococcal infections, which had been declining since the 1960s, are currently a common cause of infection. In contrast, gram-negative bacilli, resistant enterococci, and fungal infections are also on the rise.
Health care-associated infections continue to be a difficult problem, because today's hospital patients are older and more debilitated with chronic underlying diseases than in the past. Moreover, the increased use of invasive and surgical procedures, immunosuppressants, and antibiotics predisposes patients to infection and superinfection. At the same time, the growing number of personnel who can come in contact with each patient makes the risk of exposure greater.
The following measures can help prevent health care-associated infections:
❑Follow strict infection-control procedures. (See Standard precautions, pages 155 and 156, and CDC isolation precautions, page 157.)
❑Document hospital infections as they occur.
❑Identify outbreaks early, and take steps to prevent their spread.
❑Eliminate unnecessary procedures that contribute to infection.
❑Strictly follow necessary isolation techniques.
❑Observe all patients for signs of infection, especially those patients at high risk.
❑Always follow proper hand-hygiene technique and encourage other staff members to follow these guidelines as well.
❑Keep staff members and visitors with obvious infection and well-known carriers away from susceptible, high-risk patients.
❑Take special precautions with vulnerable patients — those with indwelling urinary catheters, mechanical ventilators, or I.V. lines and those recuperating from surgery.
Accurate assessment helps identify infectious diseases and prevents avoidable complications. Complete assessment consists of patient history, physical examination, and laboratory data. The history should include the patient's sex, age, address, occupation, and place of work; known exposure to illness and recent medications, including antibiotics; and date of disease onset. Signs and symptoms, including their duration and whether they occurred suddenly or gradually, should be included in the history as well as precipitating factors, relief measures, and weight loss or gain. Detail information about recent hospitalization; blood transfusions; blood donation denial by the Red Cross or other agencies; recent travel or camping trips; exposure to animals; and vaccinations. (See Immunization schedule.) If applicable, ask about possible exposure to sexually transmitted diseases or about drug abuse. Also, try to determine the patient's resistance to infectious disease. Ask about usual dietary patterns, unusual fatigue, and any conditions, such as neoplastic disease or alcoholism, that may predispose him to infection. Notice if the patient is listless or uneasy, lacks concentration, or has any obvious abnormality of mood or affect.
In suspected infection, a physical examination must assess the skin, mucous membranes, liver, spleen, and lymph nodes. Check for and make note of the location and type of drainage from any skin lesions. Record skin color, temperature, and turgor; ask if the patient has pruritus. Take his temperature, using the same route consistently, and watch for a fever, which is the best indicator of many infections. (Keep in mind that some patients, such as those who are immunocompromised, are unable to spike a fever.) Note and record the pattern of temperature change and the effect of antipyretics. Be aware that certain analgesics may contain antipyretics. With a high fever, especially in children, watch for seizures.
Check the pulse rate. Infection commonly increases the pulse rate, but some infections, notably typhoid fever and psittacosis, may decrease it. Also observe for increased respiratory rate or a change in mental status. In severe infection or when complications are possible, watch for hypotension, hematuria, oliguria, hepatomegaly, jaundice, bleeding from gums or into joints, and an altered level of consciousness. Obtain laboratory studies and appropriate cultures as ordered.
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Copyright notice for book excerpts: Copyright © 2008 Lippincott Williams & Wilkins. All rights reserved.
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More About This Book:
Title: Professional Guide to Diseases (Eighth Edition) Authors: Springhouse Publisher: Lippincott Williams & Wilkins Copyright: 2005 ISBN: 1-58255-370-X 
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