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 pathogens.

• 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 disease.

Live WormIn patient’s eye

Causes of Disease• One-half of all human diseases are

infectious.• Not all diseases are caused by

pathogens.1. inherited disease: sickle cell

anemia2. body aging (wear and tear):

osteoarthritis• Pathogens can be transmitted in 4 main

ways:1. direct contact: STD’s and influenza 2. by an object: food poisoning3. through the air: sneezing4. intermediate organisms (vectors):

malaria (spread by mosquitoes)

Causes of Disease

• Endemic diseases: diseases that are constantly present in a population like a cold.

• 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 disease

Koch’s postulates:1. pathogen must be found in host2. pathogen must be isolated and

grown in a pure culture3. 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 diseaseExceptions to Koch’s

postulates: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.