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Pathology - the scientific study of the nature of disease and its causes. A  PATHOGEN is any disease causing agent.  Quick Exercise:  How many diseases can you think of? Now watch " The A to Z of Germs... ". S ome diseases are communicable, such as:

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pathology the scientific study of the nature of disease and its causes

Pathology - the scientific study of the nature of disease and its causes

A  PATHOGEN is any disease causing agent. 

Quick Exercise:  How many diseases can you think of?

Now watch "The A to Z of Germs..."


Some diseases are communicable, such as:

anthrax, swine flu, herpes, common cold, malaria, salmonella, AIDS

Other diseases are not contagious:

cancer, lupus, arthritis, allergies

This unit will focus on the first type:  the disease, its agents, treatment and history and will cover three main areas of pathology

1.  Viruses  (virology)2.  Bacteria  (bacteriology)3.  Parasites (parasitology)


Host - organism which provides nutrients, etc. to another organism

Parasite - organism which lives at the expense of (and may even harm) its host; the parasite is generally smaller than the host and is metabolically dependent upon it

Disease - an upset in the homeostasis of the host, resulting in generation of observable changes

Symptom - evidence of damage to the host (headache)

Infectious disease - one in which detrimental changes in health of the host occur as a result of damage caused by a parasite, can be transmitted

Virulence - a measure of pathogenicity, which is the ability to cause disease (a microorganism that causes disease is virulent)

Epidemic - when a disease affects a community 

Pandemic - when a disease affects the world

1918 flu

Disease Categories

1. Food and Water borne - pathogen is in a food or water source

2. Blood Borne - carried in blood or other bodily fluids 

Sexually Transmitted - transmitted by sexual contact

3. Zoonotic- carried by animals

4. Airborne - carried by the air, often affect respiratory tract

Organizations Dealing with Health - 

Centers for Disease Control (CDC)

World Health Organization (WHO)

U.S. Department of Health and Human Services (HHS)

U.S. Army Medical Research Institute of Infectious Diseases (USAMRIID)


Koch's Postulates

If a microorganism is the causative agent of an infectious disease, it must be:

  • Present in every case of the disease, but absent from the healthy host
  • Isolated and grown in pure culture
  • Able to Cause the disease when a pure culture is inoculated into a healthy host
  • Re-isolated from the host that was inoculated with the pure culture 
Steps in Pathogenesis

To cause disease, a pathogen must:

  • Contact the host - be transmissible
  • Colonize the host - adhere to and grow or multiply on host surfaces
  • Infect the host - proliferate in host cells or tissues
  • Evade the host defense system - by avoiding contact that will damage it
  • Damage host tissues - by physical (mechanical) or chemical means

[Image: An emergency hospital ward in Kansas during the 1918 flu]


Edward Jenner (1796)

Noting the common observation that milkmaids did not generally get smallpox, Jenner theorized that the pus in the blisters which milkmaids received from cowpox (a disease similar to smallpox, but much less virulent) protected the milkmaids from smallpox.

Jenner tested his hypothesis by inoculating James Phipps, a young boy of 8 years (the son of Jenner's gardener), with material from the cowpox blisters of the hand of Sarah Nelmes, a milkmaid who had caught cowpox from a cow called Blossom

IgnazSemmelweis (1850)

Observed that women in the maternity wards died of childbed fever. He proposed that it was caused by doctors doing autopsies on the deceased women and then carrying the disease causing agent to healthy women who were in labor.

His solution: Wash your hands before delivering babies!

*The Germ Theory did not exist at this time

cholera outbreak 1854 link
Cholera outbreak 1854 link

Cholera is an infection of the small intestine caused by the bacteriumVibrio cholerae.

The main symptoms are watery diarrhea and vomiting. Transmission occurs primarily by drinking water or eating food that has been contaminated by the feces (waste product) of an infected person, including one with no apparent symptoms.

Overview of issue

It was discovered later that this public well had been dug only three feet from an old cesspit that had begun to leak fecal bacteria. A baby who had contracted cholera from another source had its nappies (diapers) washed into this cesspit, the opening of which was under a nearby house that had been rebuilt farther away after a fire had destroyed the previous structure, and the street was widened by the city. It was common at the time to have a cesspit under most homes. Most families tried to have their raw sewage collected and dumped in the Thames to prevent their cesspit from filling faster than the sewage could decompose into the soil.

After the cholera epidemic had subsided, government officials replaced the Broad Street Pump Handle. They had responded only to the urgent threat posed to the population, and afterward they rejected Snow's theory. To accept his proposal would have meant indirectly accepting the oral-fecal method transmission of disease, which was too unpleasant for most of the public to contemplate.

Louis Pasteur - developed the germ theory  and disproved spontaneous generation, in 1885 he developed the rabies vaccine

Robert Koch - Koch's postulates

Alexander Fleming - discovered penicillin, though it wasn't until much later that it was produced as an antibiotic.

Antibiotics are used to treat bacterial infections.

Jonas Salk

-polio vaccine

1980 -  WHO declared smallpox eradicated

1983 - Discovering and identification of the AIDS virus (HIV)

1985  -  First vaccine for Haemophilus influenzae type b (HiB)2006  -  First vaccine for human papillomavirus

Check out


The Germ Theory (around 1860)

  • Single most important contribution by the science of microbiology to the general welfare of the world's people
  • The theory that microorganisms may be the cause of some or all disease.
  • Key to developing the germ theory of disease was a refutation of the concept of spontaneous generation.
  • Specific aseptic techniques are employed to avoid microbial contamination
  • Method of prevention of spoilage of liquid foodstuffs - Pasteurization

*Why is this a theory and not a fact?

Malaria is an infectious disease caused by a parasite plasmodium (a one-celled parasite),that's transmitted by mosquitoes. The illness results in recurrent attacks of chills and fever, and it can be deadly.

Malaria has been virtually eradicated in countries with temperate climates, but it's still prevalent in tropical and subtropical countries in Africa, Asia, the Middle East, South America and Central America. Malaria remains one of the world's leading infectious killers, particularly of children in sub-Saharan Africa.

Evolving strains of drug-resistant parasites and insecticide-resistant mosquitoes continue to make malaria a global health problem.

Most American cases of malaria develop in travelers who have recently returned from parts of the world where malaria is widespread, particularly Africa and India. If you're traveling to malaria-endemic places, take precautions before, during and after your trip.

Treatment for malaria is with antimalarial drugs.

How do you prevent this disease?


There are actually three parallel routes, any or all of which could transform the current situation into a deadly pandemic.