Infectious Disease Process

Infectious diseases, also called communicable diseases, are never caused by one event but rather by a unique combination of events: a harmful microbe comes into contact with a susceptible host, human or animal, in the proper environment. The occurrence of an infectious disease can be blocked when any of these three elements are modified or eliminated.

Often, these microbes can live in our bodies where they do no harm. They become a problem if the body's resistance to infection is lowered; then they multiply and spread. These invading microbes may interfere with the working of the body's cells, destroy its tissues, or produce poisonous substances called toxins.

When the body is infected, its immune system comes to the defense. Specialized white cells neutralize or engulf foreign material. Some of the symptoms of an infection are caused by the fight between attacking organisms and the body's defenses.

Sometimes the invading organisms spend time growing and multiplying before any symptoms appear. This is known as the incubation period. Depending upon the organism, this period can last from one day to several years. Unfortunately, a disease may be infectious before any symptoms appear, allowing its spread to other people.

Factors Required To Produce An Infectious Disease

  1. Microbe agent- Bacteria, Fungi, Helminths, Protozoa, and Viruses
  2. Animal, human, or environmental reservoir where the agent can live, multiply and die.
  3. Organisms must leave the reservoir to spread disease. This can occur in humans through breaks in the skin or skin lesions, coughing, saliva, and secretions of the genital tract. Certain infectious diseases can be transmitted from an expectant mother across the placenta to the developing fetus.
  4. Organisms must be transmitted, either directly or indirectly, from one place to another. An organism may have more than one mode of transmission.
  5. Organisms must have a way to enter a susceptible human or animal host. Entry is similar to method of exit.
  6. There must be a susceptible host in order for disease transmission to occur. In general, people stay healthy because of their own defense mechanisms including their immune system and general defense mechanisms, such as multiple layers of skin, cough reflex, gastric juices, diarrhea and normal bacterial flora.

Deer Mouse

Infectious diseases can be spread from animals to humans from animal bites or contact with infected animal tissue, fluids, feces, or saliva. Animals can act as the reservoir, a source for the organism to survive and multiply. Rodents are reservoirs in the transmission of hantavirus (the deer mouse), bubonic plague, and Lyme disease. Animals also serve as vectors, by carrying disease causing microorganisms from one host to another. Arthropods such as ticks, mosquitoes and fleas can serve as disease vectors. Diseases that occur primarily in wild and domestic animals and can be transmitted to humans are called zoonoses.


Individual Drinking Cup Vendor

Communicable diseases can be spread indirectly by handling things that an infected person has used. Handkerchiefs, towels, bedding, diapers, drinking cups, eating utensils, toys, money and thermometers are just a few of the many objects that may carry infectious microbes. Contaminated syringes transmit HIV/AIDS and hepatitis B. Understanding of the spread of microbes led to the decline of the common drinking cup and the creation of individual drinking cup dispensers.

Microscopes & New Tools

"New truths become evident when new tools become available."
Rosalyn Yalow, Noble Laureate (1977)

The microscope was a key to the discovery of microbes and their role in disease. The human eye cannot see objects with a diameter smaller than about 1/250 inch. Most germs that cause disease are at least 50 times smaller than that. Our identification and understanding of the microbes increases with the development of each new tool or advance in technology.

Replication of a van Leeuwenhoek microscope in the University of Utrecht, ca. 1687

The microscope was invented in 1643 by Antoni van Leeuwenhoek (1632-1723), a Dutch textile businessman and self-taught scientist. This simple microscope had only one lens and could magnify up to 275 times life-size. The best lens grinder in the world in his day, van Leeuwenhoek was able to see blood cells, bacteria and other microbes.


Arcana Naturac Ope & Beneficio Exquisitissimorum Microscopiorum, 1696



Zentmayer binocular compound microscope, U.S. Army Hospital model, 1876

William Zentmayer of Philadelphia was one of the main providers of microscopes to military hospitals during and after the Civil War. A modern compound light microscope has a series of lenses and uses visible light as its source of illumination. Very small specimens can be examined, showing them in fine detail.


Microscopic Observations or Dr. Hooke Wonderful Discoveries by the Microscope, 1780

Contemporaries of van Leeuwenhoek, such as Robert Hooke, built microscopes with multiple lenses, called compound microscopes. However these early compound microscopes were of poor quality and could not be used to see bacteria. It was not until about 1830 that a better microscope was developed by Joseph Jackson Lister (father of Joseph Lister). Various improvements to Lister's microscope resulted in the modern compound microscope used in microbiology laboratories today.


Lightbox / X-ray of tuberculosis

Until the early 20th century the only way to look inside the body was to cut it open. X-rays were discovered in 1895 by Wilhelm Roentgen (1845-1923), a German scientist. Many scientists, including Marie Curie (1867-1934), worked on the application of x-rays for medical diagnosis. X-rays improved physicians' ability to diagnose tuberculosis and other respiratory infectious diseases.


National Center for Infectious Diseases Internet Home Page

Scientists around the world are using the Internet to create a system of early detection and timely response to infectious disease outbreaks using a website designed by the Federation of American Scientists, entitled Program for Monitoring Emerging Diseases (ProMED). Numerous other websites such as the National Center for Infectious Diseases and the World Health Organization serve to educate and inform about emerging infectious diseases.