1. Lesson Overview
Infectious Disease
Lesson Overview
35.1 Infectious Diseases

2. 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.”
This isn’t all that surprising, because, until microscopes were invented,
most causes of disease were invisible to the human eye!

3. Lesson Overview
Infectious Disease
Causes of Infectious Disease
What causes infectious disease?

4. Lesson Overview
Infectious Disease
Causes of Infectious Disease
What causes infectious disease?
Infectious diseases can be caused by viruses, bacteria, fungi, single-celled
eukaryotes (formerly called protists), and parasites.

5. 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.
Microorganisms were commonly called “germs,” so this conclusion was
called the germ theory of disease.

6. Lesson Overview
Infectious Disease
Agents of Disease
Infectious diseases are caused by pathogens—organisms that invade
the body and disrupt its normal functions.
Examples of pathogens are viruses, bacteria, single-celled eukaryotes,
fungi, and parasites.

7. 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.
Viruses cause the common cold, influenza, chicken pox, and warts.

8. 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.
Bacteria cause streptococcus infections, diphtheria, botulism, and
anthrax.

9. 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.
The fungus Trichophyton interdigitale causes athlete’s foot.

10. 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.
Giardia intestinalis causes infection of the digestive tract and is
transmitted in infected water.

11. 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.
Other parasitic worms include tapeworms and hookworms.

12. 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.
1. The pathogen must be isolated and grown in the laboratory in pure
culture.

13. 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.
Although there are exceptions to these rules, they remain important
guidelines for identifying the causes of new and emerging diseases.

14. 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.
Bacteria in the large intestine help with digestion and produce vitamins.

15. 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.
The “bad guys” cause problems in various ways.

16. 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.
Parasitic worms may block blood flow through blood vessels or organs,
take up the host’s nutrients, or disrupt other body functions.

17. Lesson Overview
Infectious Disease
How Diseases Spread
How are infectious diseases spread?

18. 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
contaminated water or food. Still other diseases are spread to humans
from infected animals.

19. 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
favors pathogens with adaptations that help them spread from host to host.

20. 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.
Coughing and sneezing releases thousands of tiny droplets that can
be inhaled by other people.

21. 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!
The ability of a flu virus or a tuberculosis bacterium to cause a host
to sneeze or cough is an adaptation that increases transmission of
the pathogen from one host to another.

22. 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.
They can also be transferred by contact with towels or certain kinds of
sports equipment.

23. Lesson Overview
Infectious Disease
Coughing, Sneezing, and Physical
Contact
The most important means of infection control is thorough and frequent
hand washing.
If you have a cold or flu, cover your mouth with a tissue when you
cough or sneeze, and wash your hands regularly.

24. 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.
Sexually transmitted diseases can only be completely prevented by
avoiding sexual activity.

25. 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.
HIV can be transmitted through blood or sexual contact.

26. 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.
Symptoms of these diseases often include serious diarrhea, another
adaptation that helps pathogens spread from one host to another,
especially in places with poor sanitation.

27. Lesson Overview
Infectious Disease
Contaminated Water or Food
Bacteria of several kinds are commonly present in seafood and
uncooked meat, especially ground meat.
If meats and seafood are not stored and cooked properly, illness can
result.

28. Lesson Overview
Infectious Disease
Zoonoses: The Animal Connection
Any disease that can be transmitted from animals to humans is called a
zoonosis.
Mad cow disease, severe acute respiratory syndrome (SARS), West
Nile virus, Lyme disease, Ebola, and bird flu are all zoonoses.

29. 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.
Mosquitos can transfer West
Nile virus between birds and
humans.

30. 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
comes in close contact with an infected animal’s wastes or
secretions.