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Causes of Diseases
Causes of Disease
• Disease is a change that disrupts
homeostasis in the body.
• Disease-producing agents such as
bacteria, protozoans, fungi, viruses
and other parasites are called
• The main sources of pathogens
are soil, contaminated water, and
infected people or animals.
• Any disease caused by the
presence of pathogens in the
body is called an infectious
Live Worm
In patient’s
Causes of Disease
• One-half of all human diseases are
• Not all diseases are caused by pathogens.
1. inherited disease: sickle cell anemia
2. body aging (wear and tear):
• Pathogens can be transmitted in 4 main
1. direct contact: STD’s and influenza
2. by an object: food poisoning
3. through the air: sneezing
4. intermediate organisms (vectors):
malaria (spread by mosquitoes)
Causes of Disease
• Endemic diseases: diseases that are
constantly present in a population like a
• Epidemic: occurs when many people
in a given area are afflicted with the
same disease in a short period of time.
(polio in the 1950’s)
• Carriers are people who harbor a
disease without showing any signs, yet
they can pass the disease on to others.
Procedure to establish cause of
Koch’s postulates:
1. pathogen must be found in host
2. pathogen must be isolated and
grown in a pure culture
3. pathogen from the pure culture is
placed in a healthy host (ex: rat)
4. pathogen must then be isolated from
the new host and be shown to be the
original pathogen
Procedure to establish cause of
Exceptions to Koch’s
1. some organisms have
never successfully grown in
an artificial medium
(syphilis and leprosy)
2. viral pathogens cannot be
cultured using this method
Interesting Fact
When you sneeze, all bodily functions
stop, even your heart.