Lesson Overview
Infectious Disease
Lesson Overview
Infectious Disease
THINK ABOUT IT
For thousands of years, people believed that diseases were caused by curses, evil spirits, or vapors rising from foul marshes or dead plants and animals. In fact, malaria was named after the Italian words mal aria, meaning “bad air.”
Lesson Overview
Infectious Disease
Causes of Infectious Disease
Lesson Overview
Infectious Disease
Causes of Infectious Disease
What causes infectious disease?
Lesson Overview
Infectious Disease
Causes of Infectious Disease
During the mid-nineteenth century, Louis Pasteur and Robert Koch established a scientific explanation for infectious disease.
Pasteur’s and Koch’s observations and experiments led them to conclude that infectious diseases are caused by microorganisms.
Lesson Overview
Infectious Disease
Agents of Disease
Infectious diseases are caused by pathogens—organisms that invade the body and disrupt its normal functions.
Lesson Overview
Infectious Disease
Agents of Disease
Viruses are nonliving particles that replicate by inserting their genetic material into a host cell and taking over many of the host cell’s functions.
Lesson Overview
Infectious Disease
Agents of Disease
Bacteria cause disease by breaking down the tissues of an infected organism for food, or by releasing toxins that interfere with normal activity in the host.
Lesson Overview
Infectious Disease
Agents of Disease
Different types of fungus may infect the surface of the skin, mouth, throat, fingernails and toenails. Dangerous infections may spread from the lungs to other organs.
Lesson Overview
Infectious Disease
Agents of Disease
The single-celled eukaryote Plasmodium causes malaria, a very damaging infectious disease.
The single-celled eukaryote Trypanosoma brucei feeds off
nutrients in its host’s blood and causes African sleeping sickness.
Both Plasmodium and Trypanosoma brucei are spread to human by insects.
Lesson Overview
Infectious Disease
Agents of Disease
People may be infected with the roundworm Trichinella spiralis
from eating infected pork.
The flatworm Schistosoma mansoni can be contracted by people working in rice paddies.
Lesson Overview
Infectious Disease
Koch’s Postulates
Koch’s studies with bacteria led him to develop rules for identifying the microorganism that causes a specific disease. These rules are known as Koch’s postulates.
1. The pathogen must always be found in the body of a sick organism and should not be found in a healthy one.
Lesson Overview
Infectious Disease
Koch’s Postulates
Koch’s studies with bacteria led him to develop rules for identifying the microorganism that causes a specific disease. These rules are known as Koch’s postulates.
3. When the cultured pathogens are introduced into a healthy host, they should cause the same disease that infected the original host.
4. The injected pathogen must be isolated from the second host. It should be identical to the original pathogen.
Lesson Overview
Infectious Disease
Symbionts vs. Pathogens
Most microorganisms that live and grow in the human body are symbionts that are either harmless or actually beneficial.
Yeast and bacteria grow in the mouth and throat without causing trouble.
Lesson Overview
Infectious Disease
Symbionts vs. Pathogens
What’s the difference between harmless microorganisms and pathogens that cause disease?
The “good guys” obtain nutrients, grow, and reproduce without disturbing normal body functions.
Lesson Overview
Infectious Disease
Symbionts vs. Pathogens
Some viruses and bacteria directly destroy the cells of their host.
Other bacteria and single-celled parasites release poisons that kill the host’s cells or interfere with their normal functions.
Lesson Overview
Infectious Disease
How Diseases Spread
Lesson Overview
Infectious Disease
How Diseases Spread
How are infectious diseases spread?
Some diseases are spread through coughing, sneezing, physical contact, or exchange of body fluids. Some diseases are spread through
Lesson Overview
Infectious Disease
How Diseases Spread
Pathogens are often spread by symptoms of disease, such as sneezing, coughing, or diarrhea.
In many cases, these symptoms are changes in host behavior that help pathogens spread and infect new hosts.
If a virus infects only one host, that virus will die when the host’s immune system kills it or when the host dies. For that reason, natural selection
Lesson Overview
Infectious Disease
Coughing, Sneezing, and Physical
Contact
Many bacteria and viruses that infect the nose, throat, or respiratory tract are spread by indirect contact.
Lesson Overview
Infectious Disease
Coughing, Sneezing, and Physical
Contact
Those droplets also settle on objects such as doorknobs. If you touch those objects and then touch your mouth or nose, you can transfer the pathogens to a new home!
Lesson Overview
Infectious Disease
Coughing, Sneezing, and Physical
Contact
Other pathogens, including drug-resistant staphylococci that cause skin infections, can be transferred by almost any kind of body-to-body contact.
Lesson Overview
Infectious Disease
Coughing, Sneezing, and Physical
Contact
The most important means of infection control is thorough and frequent hand washing.
Lesson Overview
Infectious Disease
Exchange of Body Fluids
Some pathogens require specific kinds of direct contact to be transmitted from host to host.
A wide range of diseases, including herpes, gonorrhea, syphilis, and chlamydia, are transmitted by sexual activity. Therefore, these diseases are called sexually transmitted diseases.
Lesson Overview
Infectious Disease
Exchange of Body Fluids
Other diseases, including certain forms of hepatitis, can be transmitted among users of injected drugs through blood from shared syringes.
Lesson Overview
Infectious Disease
Contaminated Water or Food
Many pathogens that infect the digestive tract are spread through water contaminated with feces from infected people or other animals.
Contaminated water may be consumed, or it may carry pathogens onto fruits or vegetables. If those foods are eaten without being washed thoroughly, infection can result.
Lesson Overview
Infectious Disease
Contaminated Water or Food
Bacteria of several kinds are commonly present in seafood and uncooked meat, especially ground meat.
Lesson Overview
Infectious Disease
Zoonoses: The Animal Connection
Any disease that can be transmitted from animals to humans is called a zoonosis.
Lesson Overview
Infectious Disease
Zoonoses: The Animal Connection
Sometimes an animal carries, or transfers, zoonotic diseases from an animal host to a human host.
These carriers, called
vectors, transport the pathogen but usually do not get sick
themselves.
Lesson Overview
Infectious Disease
Zoonoses: The Animal Connection
In other cases, infection may occur when a person is bitten by an infected animal, consumes the meat of an infected animal, or