Infectious and Noninfectious Conditions

Most diseases today are multifactorial, or caused by the interaction of several factors from outside and inside the person.  For a disease to occur, the host must be susceptible, meaning that the immune system must be in a weakened condition; an agent capable of transmitting a disease must be present; and the environment must be hospitable to the pathogen in terms of temperature, light, moisture, and other requirements.  Other risk factors also apparently increase or decrease levels of susceptibility.

• Pathogen – a disease-causing agent

• Virulent – said of organisms able to overcome host resistance and cause disease

• Multifactorial disease – disease caused by interactions of several factors

• Uncontrollable risk factors – those that increase one’s susceptibility and over which one might have little or no control.  

Some of the most common uncontrollable risk factors over which humans are helpless include:

• Heredity - choose your parents wisely

• Aging - beats the alternative

• Gender - in most cases, we are what we are

• Ethnic membership - again, thanks to mom & dad

• Environmental conditions - some are more mobile than others

Risk factors over which one can exert some measure of control include various behavioral choices:

• smoking - choosing to start and continue

• exercise - choosing to start and continue

• diet - choosing what to eat, daily

• alcohol - choosing to start and continue

Pathogens:  Routes of Invasion

Pathogens enter the body in several ways.  They may be transmitted by direct contact between infected persons, or by indirect contact.  The hands are probably the greatest source of transmission.  Pathogens are also transmitted by airborne contact, either through inhaling the droplet spray from a sneeze or breathing in air that carries a particular airborne pathogen, or you may become the victim of a food-borne infection if you eat something contaminated by microorganisms.  Your best friend may be the source of animal-borne pathogens.  Dogs as well as cats, livestock, and wild animals can spread numerous diseases through their bites or feces or by carrying infected insects into your living areas (Zoonosis:  defined as any infection or infectious disease transmissible from animals to humans).  Water-borne diseases are transmitted directly from drinking water and indirectly from foods washed or sprayed with water containing their pathogens.  These pathogens can also invade your body if you wade or swim in contaminated streams, lakes, and reservoirs. 

Bacteria are the single-celled organisms that are plantlike in nature by lack chlorophyll.  There are three major types of bacteria:  cocci. bacilli, and spirilla.  Bacteria may be viewed under a standard light microscope.  Although there are several thousand species of bacteria, only approximately 100 cause diseases in humans.  In many cases, it is not the bacteria themselves that cause disease but rather the poisonous substances, called toxins, that they produce.  Some of these toxins are extremely powerful.  Bacterial infections can take many forms.  The following are the most common:

• Staphylococcal infections

• Streptococcal infections

• Pneumonia

• Legionnaire’s disease

• Tuberculosis

• Periodontal diseases

Viruses are the smallest of the pathogens, being approximately 1/500^th the size of a bacteria.  Because of their tiny size, they are visible only under an electron microscope and were therefore not identified until this century.  By the 1960s, viruses were being effectively grown outside the body in tissue cultures.  At present over 150 viruses are known to cause diseases in humans.

Drug treatment for viral infections is limited.  Drugs powerful enough to kill viruses also kill the host cells, although there are some drugs available that block stages in viral reproduction without damaging the host cells.  We have another form of virus protection within our own bodies.  When exposed to certain viruses, the body begins to produce a protein substance known as interferon.  Interferon does not destroy the invading microorganisms but sets up a protective mechanism to aid healthy cells in their struggle against the invaders.  Although interferon research is promising, not all viruses stimulate interferon production.  Some types of virus activity include:

• The common cold

• Influenza

• Infectious mononucleosis

• Hepatitis

• Mumps

• Chicken pox

• Measles

 Other pathogens include:

• Fungi – hundreds of species of fungi, multi- or unicellular primitive plants, inhibit our environment and serve useful functions.  Some species of fungi can produce infections, e.g., athlete’s foot, ringworm, and jock itch are examples of fungal diseases.  Keeping the affected area clean and dry plus treatment with appropriate medications will generally bring prompt relief from these infections.

• Protozoa – are microscopic, single-celled organisms that are generally associated with tropical diseases such as African sleeping sickness and malaria.  Although these pathogens are prevalent in the developing countries of the world, they are largely controlled in the U.S.  The most common protozoal disease in the U.S. is trichomoniasis, a sexually transmitted infection.  The “trich” infection may cause a foamy, yellowish discharge with an unpleasant odor that may be accompanied by a burning sensation, itching, and painful urination.  Although usually transmitted by sexual contact, the trich organism may be easily spread by toilet seats, wet towels, or other items that have discharged fluids on them.  One can also contract trichomoniasis by sitting naked on the bench of the dressing room of your local health spa or locker room.  Treatment includes oral metronidazole, usually given to both sexual partners to avoid the possible “ping-pong” effect or repeated cross-infection so typical of sexually transmitted infections.  A common water-borne protozoan disease in many regions of the country is giardiasis.  Persons who drink or are exposed to the giardia pathogen may suffer symptoms of intestinal pain and discomfort weeks after infection.  Protection of water supplies is the key to prevention.

• Parasitic worms – are the largest of the pathogens.  Ranging in size from the relatively small pinworms typically found in children to the relatively large tapeworms found in all forms of warm-blooded animals, most parasitic worms are more a nuisance than a threat.  Of special note today are the new forms of worm infestations commonly associated with eating raw fish in Japanese sushi restaurants.  Cooking fish and other foods to temperatures to kill the worms or their eggs is an effective means of prevention.

Your Body’s Defenses:  Keeping You Well

Although all of the pathogens described in the preceding section pose a threat if they take hold in one’s body, the chances that they will take hold are actually quite small.  To do so, they must overcome a number of effective barriers, many of which were established in your body before your birth.

• Physical and chemical defenses –

1.      Perhaps our single most critical early defense system is the skin.  Layered to provide an intricate web or barriers, the skin allows few pathogens to enter.

2.      Enzymes, complex proteins manufactured by the body that appear in the body secretions such as sweat, provide additional protection, destroying microorganisms on skin surfaces by producing inhospitable pH levels.

3.      Another protection is our frequent slight elevations in body temperature, which create inhospitable environment for many pathogens.  Only when there are cracks or breaks in the skin can pathogens gain easy access to the body.

4.      The linings of the body provide yet another protection against pathogens.  Mucous membranes in the respiratory system and other linings of the body trap and engulf invading organisms.  Cilia, hairlike projections in the lung and respiratory tract, sweep unwanted invaders toward boy openings, where they are expelled.

5.      Any invading organism that manages to breach these initial lines of defense faces a formidable specialized network of defenses thrown up by the immune system.

• The Immune System –

Immunity is a condition of being able to resist a particular disease by counteracting the substance that produces the disease.  Any substance capable of triggering an immune response is called an antigen.  An antigen can be a virus, a bacterium, a fungus, a parasite, or a tissue or cell from another individual.  When invaded by an antigen, the body responds by forming substances called antibodies that are matched to the specific antigen much as a key is matched to a lock.  

Autoimmune diseases – although white blood cells and the antigen-antibody response generally work in our favor by neutralizing or destroying harmful antigens, the body sometimes makes a mistake and targets its own tissue as the enemy, builds up antibodies against that tissue, and attempts to destroy it.  This is known as autoimmune disease (auto means self).  Common examples of this type of disease are rheumatoid arthritis, lupus erythematosus, and myasthenia gravis.

• Fever –

If an infection is localized, pus formation, redness, swelling, and irritation often occur.  These symptoms indicate that the invading organisms are being fought systematically.  Another indication is the development of a fever, or a rise in body temperature above the norm of 98.6 degrees F.  fever is frequently caused by toxins secreted by pathogens that interfere with the control of body temperature.  Elevations of body temperature by even 1 or 2 degrees provide an environment that destroys some types of disease-causing organisms.  Also, as body temperature rises, the body is stimulated to produce more white blood cells which destroy more invaders.

• Pain –

Although pain is not usually thought of as a defense mechanism, it plays a valuable role in the body’s response to invasion.  Pain is generally a response to injury.  Pain may be either direct, caused by the stimulation of nerve endings in an affected area, or referred, meaning it is present in one place while the source is elsewhere.  An example of referred pain is the pain in the arm or jaw often experienced by someone having a heart attack.

• Vaccines –

Our natural defense mechanisms are our strongest allies in the battle against disease, being with us from birth until death.  There are periods in our life, however, when either invading organisms are too strong or our own natural immunity is too weak to protect us from catching a given disease.  It is at such times that we need outside help in developing immunity to an invading organism.  Such assistance is generally provided in the form of a vaccination, which consists of killed or weakened versions of disease microorganisms of antigens that are similar to but far less dangerous than the disease microorganism.  Vaccines are given orally or by injection and this form of artificial immunity is termed acquired immunity, in contrast to natural immunity, which a mother passes to her fetus via their shared blood supply.

Emerging Diseases:  Challenges to World Health

Take a look at each of these:  Bugs in the News...Also, "What the heck is...."?

1. Mad Cow Disease

2. Ebola Virus

3. Hantavirus

4. E.Coli Bacteria

5. Flesh-eating Strep

6. Cholera

7. Dengue Fever

8. Cryptosporidium

This page was last updated on 10/09/03

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