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
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
- Microbe agent- Bacteria, Fungi, Helminths, Protozoa, and Viruses
- Animal, human, or environmental reservoir where the agent can
live, multiply and die.
- 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.
- Organisms must be transmitted, either directly or indirectly,
from one place to another. An organism may have more than one
mode of transmission.
- Organisms must have a way to enter a susceptible human or animal
host. Entry is similar to method of exit.
- 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.
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.
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
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.