There are thousands of infectious agents that can cause human disease. Although the body is extraordinarily adaptive in its responses to such agents, sometimes its
preventative measures fail, resulting in disease. A subclinical infection occurs when the body's defensive mechanisms are effective, resulting in no apparent clinical symptoms. When infection persists to cause disease, it is called an acute or chronic infection.
There are four major classes of organisms that infect the human body:
- Viruses: microscopic agents that consist of genetic material coding for the virus's reproduction enclosed in a protective protein coat or lipid membrane. Viruses are obligate intracellular parasites; they cannot replicate without first infecting a cell and exploiting its reproductive capabilities.
- Bacteria: microscopic prokaryotic organisms (lacking a nuclear membrane, mitochondria, and other organelles). Two major classes include gram-positive bacteria (surrounded by a protective cell wall) and gram-negative bacteria (surrounded by an outer lipid membrane).
- Fungi: eukaryotic organisms (containing distinct organelles and a nucleus enclosed by a nuclear membrane). Fungi can be unicellular (e.g., yeast) or multicellular (e.g., mold).
- Parasites: eukaryotic organisms ranging from microscopic, unicellular protozoa to macroscopic arthropods and worms.
Infectious organisms are found everywhere on Earth—in extremes of hot and cold; in acidic and alkaline environments; in air, soil, and water; in our bodies, and on our skin. The human body is colonized by numerous types of bacteria (called normal flora) that reside in the stomach, intestines, colon, upper respiratory tract, and
B cells—White blood cells responsible for the production of antibodies.
Ciliated cells—Cells with hair-like structures that help flush out foreign particles from the human body.
Complement system—Proteins that activate inflammation response and recruit white blood cells to the site of infection.
Endogenous infection—Infection caused by the normal flora of the human body.
Eukaryote—An organism whose cells contain a true nucleus bound by a membrane.
Exogenous infection—Infection caused by microbes found external to the human body.
Normal flora—Types of bacteria and other organisms that colonize the human body without normally causing disease.
Obligate intracellular parasites—Microbes that must remain inside of a cell in order to survive and replicate.
Phagocytosis—Engulfment and digestion of foreign particles and cells by phagocytic cells such as neutrophils and macrophages.
Prokaryote—A cell that contains no true nucleus or membrane-bound organelles.
T cells—White blood cells responsible for activating and controlling immune response.
on the skin. Ordinarily, normal flora aids in food digestion, protection against disease, and various other functions. Exogenous infections occur when organisms found outside of the body cause disease, while endogenous infections are caused by the normal flora colonizing sterile tissue sites.
There are countless ways in which an individual can become infected with an infectious organism. The mode of transmission depends largely on the type of organism, its size, its structure, its vector (who transmitted it), and other factors. Some common ways that infectious agents are transmitted (and examples of such agents) are:
- inhalation (Mycobacterium tuberculosis; influenza viruses; Histoplasma capsulatum, a fungus that causes pneumonia)
- ingestion (Salmonella, Vibrio, Giardia and Listeria species; Escherichia coli)
- penetration of skin (Clostridium tetani, causative agent of tetanus; Staphylococcus aureus; hepatitis C virus [HCV])
- sexual transmission (human immunodeficiency virus [HIV]; Neisseria gonorrhoeae; Chlamydia trachomatis)
- zoonoses or animal contact (flaviviruses; rabies virus; Yersinia pestis, causative agent of bubonic plague)
- mother-to-child (Rubella virus or German measles; herpes simplex virus [HSV]; varicella-zoster virus or chicken pox)
Response to infection
The human body has three basic means of defense against invading microorganisms: natural barriers, innate non-specific immunity, and antigen-specific immunity. Each protective measure acts at a different time point in infection and varies according to the type of infectious agent.
NATURAL BARRIERS. The first barriers against infection are the skin and mucous membranes (the inner lining of the mouth, nose, vagina, urethra, and upper respiratory tract). Besides providing a physical barrier against the entry of infectious agents, these tissues are inhospitable environments for invading microbes. For example, mucus (a secretion made of protein and sugar molecules) in the upper respiratory tract can trap infectious particles before they go on to colonize the lung; ciliated cells (with hair-like structures on their surface) help flush the particles out of the respiratory tract to be expelled. The gastrointestinal tract (including the stomach and intestines) and the urinary tract (including the bladder and kidneys) secrete fluids such as gastric juice and bile that create hostile conditions for infectious agents.
The temperature of the human body (normally 98.6°F or 37°C) is itself a mechanism of evading infection. A major elevation of body temperature (i.e., fever) can slow or prevent the colonization and spread of many microbes and increase the efficiency of immune response.
INNATE NON-SPECIFIC IMMUNE RESPONSE. When an infectious agent is able to evade natural barriers and enter the body, the first responses to its presence are non-specific protective responses. For example, the presence of certain microbial surface molecules activates the complement system (proteins that activate inflammation response and recruit white blood cells to the site of infection).
ANTIGEN-SPECIFIC IMMUNE RESPONSE. If non-specific immunity fails to slow or prevent the spread of a microorganism, another line of defense may be used: antigen-specific immunity. Two classes of white blood cells have a large role in specific immune response; these are B cells (or B lymphocytes) and T cells (or T lymphocytes).
B cells are responsible for the production of antibodies, also called immunoglobulins. Antibodies bind specifically to a foreign particle (called an antigen) so that once antibodies have been produced against a particular invader, the immune system can react more rapidly if that invader enters the body again. Antibodies can also enhance phagocytosis, neutralize toxins, inhibit the binding of microorganisms to human cells, and activate the complement system.
There are two main types of T cells: helper T cells (CD4 type) and cytolytic and suppressor T cells (CD8 type). Helper T cells activate and control immune response by stimulating B cells to produce antibodies. Receptors on the surface of cytolytic T cells recognize cells with surface antigens; the cell is then killed. Suppressor T cells help regulate immune response.
In some cases immune response is over-stimulated, resulting in extensive tissue damage and systemic effects. An example is toxic shock syndrome (TSS), a disease that results from infection with Staphylococcus aureus. Upon infection the bacteria produces a toxin that over-stimulates immune response. The result is a proliferation of T cells and over-secretion of cytokines (small proteins that act as signals between cells of the immune system). The clinical manifestations of this disease are devastating: symptoms start with fever and hypotension (low blood pressure) and may progress to multiple organ failure and desquamation of the skin (extensive peeling or scaling).
In the brochure "An Ounce of Prevention: Keeps the Germs Away," (2000) the Centers for Disease Control and Prevention (CDC) identified some simple and inexpensive means of preventing the spread of infectious diseases. These include:
- Wash hands frequently.
- Clean and disinfect.
- Handle and prepare foods safely.
- Get immunized.
- Do not take unnecessary antibiotics (e.g., for viral infections).
- Keep pets healthy.
- Avoid contact with wild animals.
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National Center for Infectious Diseases. Mailstop C-14, 1600 Clifton Road, Atlanta, GA 30333. <http://www.cdc.gov/ncidod/>.
"Biology of Infectious Disease." In The Merck Manual of Diagnosis and Therapy, on-line. 2001. Merck & Co., Inc. <http://www.merck.com/pubs/mmanual/section13/chapter150/150a.htm>.
Stephanie Islane Dionne