Inflammation is the body’s immunologic response to tissue damage caused by the invasion of foreign bodies, microorganisms, or harmful chemicals. This invasion may result from trauma; physical agents (temperature extremes, radiation) or chemical agents (poisons, venoms); allergens; and diseaseproducing, or pathogenic, organisms (bacteria, viruses, fungi). The process begins with the entry of the physical irritant and ends with healing. Inflammation occurs when microorganisms gain entry into the body, most likely through a break in the skin. How well the body responds to inflammation depends on:

  1. an individual’s general health, nutritional state, and age
  2. tissue factors
  3. type of physical irritant

Inflammation may be acute or chronic. In its acute phase, there is redness, swelling, pain, heat, and maybe even loss of function. At a site of injury, there are a large number of polymorphonuclear leukocytes, which are white blood cells (WBCs) that possess a nucleus composed of 200 or more lobes or parts. Examples of acute inflammation include insect bites, mild burns, and minor abrasions and cuts. The inflammation may persist, spread to adjacent or distant tissue, and become chronic. In chronic inflammation, there is an increase in the number of lymphocytes, monocytes, and plasma cells.

When microorganisms gain entry into the body, they release a toxin that causes the capillaries of the host to become permeable and allow access to WBCs—hence, the redness, swelling, heat, and pain. Factors that help in abating the inflammatory response include topical applications of ice packs, adequate hydration and nutrition, rest, and good blood supply. Nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen (Aleve and Advil) are useful in managing inflammation. Inflammation is a beneficial biological response in most instances; however, if it becomes chronic, inflammation can be debilitating, as is the case, for example, in rheumatoid arthritis. Whatever the cause of inflammation, it is the body’s protective response.

Infection is the invasion and multiplication of pathogenic or disease-producing microorganisms in the body. Most microorganisms in the body are nonpathogenic and, in fact, are often necessary to maintain homeostasis, a state of stability that the body tries to maintain even though it is exposed to continually changing outside forces. When one or more of the requisite factors in the infectious process are present, a microorganism can become a potential pathogen.

People serve as hosts for organisms, as do animals. A host does not necessarily have to be “diseased” or “sick” but simply serves as a reservoir for the microorganisms. Transmission can be through exposure to a host’s coughing or sneezing, through touching something contaminated by the infected host, or through direct contact with the microorganism. If the receiving host is not susceptible, then the microorganism has little chance of becoming a pathogen. The susceptible host, however, may have low resistance or provide the microorganism with an unusual means of entry, such as an open wound.

Whenever a pathogenic microorganism finds a suitable environment for growth in an appropriate host, disease may result. Growth factors for microorganisms vary and include the presence or absence of oxygen, a ready source of food, an optimal temperature, moisture, and darkness. Microorganisms, including those that cause disease, can be classified into six general groups: fungi, rickettsiae, protozoa, viruses, bacteria, and parasites.


This group includes yeasts and molds that may be present in the soil, air, and water. Only a few species cause disease. Fungal diseases, called mycoses, usually develop slowly, are resistant to treatment, and are rarely fatal. The more common mycoses include histoplasmosis, coccidioidomycosis, and thrush; tinea corporis, or ringworm; and tinea pedis, or athlete’s foot.


FIGURE. Fungi. (From Scanlon, VC, and Sanders, T: Essentials of Anatomy and Physiology, ed. 5. FA Davis, Philadelphia, 2007, p 513, with permission.)


This group of bacteria-like organisms live parasitically inside living cells. They are transmitted through bites from infected lice, fleas, ticks, mosquitoes, and mites. Rickettsial diseases include Rocky Mountain spotted fever (ICD-9: 082.0), typhus (ICD-9: 081.x), and trench fever (ICD-9: 083.1). These diseases are more likely to occur where unsanitary conditions prevail.


These single-celled organisms have animal-like characteristics. Malaria (ICD-9: 084.x), amebic dysentery (ICD-9: 006.x), and African sleeping sickness (ICD-9: 086.5) are examples of protozoan diseases.

Trichomonas vaginalis is a protozoon that causes trichomoniasis or vaginitis, a disease fairly common among women.


FIGURE. Protozoa. (From Scanlon, VC, and Sanders, T: Essentials of Anatomy and Physiology, ed. 5. FA Davis, Philadelphia, 2007, p 514, with permission.)


These are the smallest microorganisms, visible only through the use of electron microscopy. Figure illustrates common viruses and compares the size of the three viruses with that of the Escherichia coli bacillus. Viruses are independent of host cells, they are difficult to isolate, and few respond to drug therapy. Viruses may remain dormant in a host for long periods before becoming active. Viral infections include the common cold, West Nile virus, measles, mumps, rabies (ICD-9: 071), chickenpox, herpesviruses, poliomyelitis, hepatitis, influenza, and certain types of pneumonia and encephalitis.


FIGURE. Viruses: representative shapes and relative sizes. (From Scanlon, VC, and Sanders, T: Essentials of Anatomy and Physiology, ed. 5. FA Davis, Philadelphia, 2007, p 511, with permission.)


There are many varieties of these single-celled organisms. Most are nonpathogenic and useful. Bacteria, including those that cause disease, are classified according to their shape.

  • Bacilli are rod-shaped bacteria. Diseases caused by bacilli include tuberculosis, whooping cough, tetanus, typhoid fever, and diphtheria.
  • Spirilla are spiral-shaped bacteria. Diseases caused by spirilla include syphilis and cholera (ICD-9: 001.x).
  • Cocci are dot-shaped bacteria. Diseases caused by cocci include gonorrhea, meningitis, tonsillitis (ICD-9: 463), bacterial pneumonia, boils (ICD-9: 680.x), scarlet fever (ICD-9: 034.x), sore throats (ICD-9: 462), and certain skin and urinary infections.


FIGURE. Bacteria (magnification ϫ2000). (From Scanlon, VC, and Sanders, T: Essentials of Anatomy and Physiology, ed. 5. FA Davis, Philadelphia, 2007, p 509, with permission.)


This is a group of host-requiring organisms that include external and internal parasites. Helminthes (ICD-9: 128.9) are wormlike internal parasites that are typically transmitted from person to person via fecal contamination of food, water, or soil. Three classes of helminthes may infect humans.

  • Pinworms (ICD-9: 127.4) are the most common worm infection in the United States. They look like small threads about the size of a staple and often live in the human colon and rectum. During an individual’s sleep, female pinworms leave the intestine via the anus to deposit their eggs on the surrounding skin tissue, causing itching and restlessness.
  • Tapeworms (ICD-9: 123.x) are long and narrow, as their name indicates, and they depend on two hosts, one human and one animal, from the development of the egg to the larva to the adult. The easiest way to remember their names is by the name of the animal that acts as the second host: that is, beef tapeworm (ICD-9: 123.2), pork tapeworm (ICD-9: 123.0), fish tapeworm (ICD-9: 123.4), and dog tapeworm (ICD-9: 123.8). Intestinal infection occurs when raw or contaminated meat or fish is eaten.
  • Flukes (ICD-9: 121.x) are small, leaf-shaped, flat, nonsegmented worms. Fluke infection occurs when eating uncooked fish, plants, or animals from water infested with flukes.


FIGURE. Helminths. (From Scanlon, VC, and Sanders, T: Essentials of Anatomy and Physiology, ed. 5. FA Davis, Philadelphia, 2007, p 515, with permission.)