Infectious Diseases
Copyright 2010. PEER.tamu.edu
Think about it:
• What infectious disease
have you had?
• Can you name an
infectious disease that
causes people to die?
• What is an infectious
disease that has been in
the news recently?
Epidemiology
• The branch of medical science dealing with the
transmission and control of disease.
• There are human physicians that study
epidemiology and also veterinarians that study
animal epidemiology.
Infectious
Diseases are
Caused by
Microbes
What’s a
microbe?
What is a Microbe?
• Microbes are microscopic
organisms that can exist
almost anywhere. Different
microbes have different
habitat preferences, ranging
from extreme heat to
extreme cold. Some
microbes need oxygen and
some do not.
• Most microbes can survive
in a large variety of habitats,
but they can only thrive in a
few habitats.
• We even have microbes in
our bodies--some help us out
and some hurt us.
You have to have a microscope
to see microbes!
Microbes Can Multiply Fast!
Typical growth curve of an undisturbed
population of microbes at normal temperatures
(about 40 – 100 degrees F)
Biotic and Abiotic Factors in the
Environment:
•Microbes require several biotic (living)
and abiotic (non-living) factors to be
present in the environment to be able to
survive.
•Can you name one biotic factor bacteria
require?
•Can you name two abiotic factors
bacteria require?
Using what
you know
about biotic
and abiotic
factors,
Can You
Explain:
 Why growth becomes stationary?
 Why the microbes eventually die?
Common Types of Microbes
Viruses*
Bacteria
Prion*
Fungi
*See slide notes
Protozoa
Microbes and Disease
• Some microbes
cause disease and
some don’t.
Infectious agents are
microbes that
can cause disease.
• Microbes that cause
disease are called
infectious agents, we
commonly call them
“germs” or “bugs.”
Let’s go over the types of Microbes:
Bacteria
• Bacteria are unicellular (one-celled) and
prokaryotic (they don’t have a nucleus).
Bacteria can
look like
spheres, rods,
or spirals.
• Bacteria are heterotrophic (they must
consume substances to get energy to
survive).
• They are in the Domain Eubacteria and the
Kingdom Eubacteria.
• There are countless numbers of bacteria on
the Earth but less than 1% of them cause
disease in humans.
Source: NSF.gov
• Bacteria can live in a vast range of places, but
need energy sources to thrive.
Prions
• A prion is an infectious particle (not
a cell) made from an abnormally
folded protein found on the surfaces
of nerve cells. They are not
classified into a Domain or Kingdom
of living organisms. There is
controversy over whether to classify
them as microbes, but they are
infectious agents.
• Prions are highly resistant to heat,
radiation, and disinfectants.
• The best known prion forms holes in
brain tissue, making the brain look
like Swiss cheese. The prion causes
mad-cow disease and may cause
some forms of Alzheimer's
Disease.
Viruses
• A virus is a microbe that consists of
a piece of genetic material (RNA)
housed within a protective coat.
Viruses are not made of cells. They
are not classified into a Domain or
Kingdom of living organisms.
• The virus reproduces by hijacking
the cell of another organism (host)
and getting the host cell to
reproduce more viruses.
• Most viruses cause disease and are
specific as to which type of cell they
will attack.
Break Time
Discuss:
What is the difference
between living and non
living?
Protozoa
• Protozoa are unicellular (onecelled) eukaryotic (have a nucleus)
microbes that can be parasites or
predators of other microbes.
• Most need a moist environment to
live. They are heterotrophic (they
must consume substances to get
energy to survive).
Yuck!
Protozoa found in human stool
sample
• They are in the Domain Eukaryota
and Kingdom Protista
• Usually cause disease in humans.
• Protozoa can be helpful to other
animals
Giardia
Fungi
• A multi-cellular (manycelled) eukaryote (has a
nucleus in cells) microbe that
is much larger than the other
microbes.
• They are heterotrophic (they
must consume substances to
get energy to survive).
• They belong to the Domain
Eukarya and Kingdom Fungi.
• Only about 1/2 of all fungi
cause disease in humans.
• Yeast is a fungus that is used
to make bread and cheese for
us!
Quick Check #1:
1. What is a microbe?
2. Name five kinds of microbes.
3. How are these five kinds of microbes
alike?
4. How are these five kinds of microbes
different?
How Can an Infectious
Agent Attack Me?
• Infectious agents can
enter through air,
food, water, sexual
interactions, skin
contact, blood
transfusions, etc.
• The body’s reaction to
an infection can vary
from a mild discomfort
to death.
Infectious
Agent
For more on the immune
system, click here
Species Specificity
I can transmit
Brucellosis
We can transmit lots
of infectious agents
including arena
viruses and hantavirus.
I can transmit
Ebola virus!
• Some infectious
diseases of animals
can be transferred
to humans.
• These are called
zoonotic diseases.
• All mammals can
transmit rabies but
raccoons and
skunks are the
most common
carriers.
Think about it:
Where Do Infectious Agents Hide
When Not Infecting You?
The soil
Bodies of water
Surfaces like desks and tables
People’s skin
In the air
On certain animals
Where are
those
microbes?
Do you know the difference
between “infectious” and
“contagious?”
Infectious: microbe can invade
the body
Contagious: microbe can be
spread from one person to
another.
Quick Check #2
1. How can microbes get in the body to
cause infection?
2. What is an infectious disease that can
be transmitted from an animal to a
human called?
3. Where are microbes commonly found?
4. What’s the difference between being
infected and being contagious?
What are the Main Types of
Infectious Diseases?
Digestive Diseases
Respiratory Diseases
Liver Diseases
Sexually Transmitted Diseases
Skin Diseases
Some Types of Infectious
Diseases:
Type of Disease
What Microbe
Usually Causes
It
A Few of the
Known
Symptoms
Common Types
Respiratory
Diseases
Bacteria and
Viruses
Coughing,
congestion,
fluid in lungs
Bronchitis,
Pneumonia,
Cold, Flu
Digestive
Diseases
Viruses,
Bacteria and
Protozoa
Nausea,
vomiting,
diarrhea,
cramps or pains
Botulism (food
poisoning)
Stomach “flu”
(gastroenteritis)
Liver Diseases
Viruses
Fatigue, poor
appetite,
jaundice
Hepatitis
Sexually
Transmitted
Diseases
Viruses and
Bacteria
Can vary, from
mild rash to
death
Chlamydia,
Herpes, AIDS
Skin Diseases
Fungi and
bacteria
Rash, itching,
redness
Athletes foot,
acne
Food Poisoning is a Disease
Caused by Infectious Agents
Prevent food poisoning by
stopping microbes from
reproducing: keep hot foods
hot and cold foods cold.
Food Poisoning
• Botulism- collapse,
respiratory failure, and
death. (Caused by
improper canning
methods)
• Classical food poisoning
can be prevented by better
food storage and handling
techniques.
• Outbreaks usually occur at
picnics, school cafeterias,
or anywhere where the
food is not handled
properly or not kept
refrigerated.
Symptoms
• nausea
• vomiting
• abdominal cramps
• fever
• diarrhea
• See also our
curriculum on the
Digestive System
Which Foods Are a Problem?
• Almost all foods can
carry infectious
agents.
• Hamburgers, potato
salad, cold cuts, hot
dogs, soft cheeses,
eggs, and any raw
meat are favorite
places where
microbes can grow
and become likely to
infect.
Infectious Agents Can Be
Deadly
• Infectious diseases
cause more deaths
worldwide than any
other single cause.
• Infectious diseases
account for over 56%
of deaths in developing
countries.
• However, these
diseases account for
only 8% of deaths in
rich countries.
Not all infectious disease are deadly;
Acne is an Infectious Disease!
• The pimples are infections of
the skin.
• The skin makes oil from
sebaceous glands in the skin.
Too much of this oil clogs
pores and allows bacteria to
grow and multiply.
Acne is not
contagious, but it is
infectious
• White blood cells rush to
fight the infection. The blood
cells die and become pus.
Different infectious diseases
require different approaches for
prevention and control.
But for any disease, there are
three key steps for dealing
with it.
Three Key Steps
reak the cycle of transmission
ill the infectious agent
ncrease host resistance
Do you have some ideas on how to do these three things?
Quick Check #3
1. List the main types of infectious
diseases.
2. Give an example of a deadly infectious
disease.
3. Give an example of an infectious disease
that is not deadly.
4. What are the three key steps for dealing
with infectious disease?
Activity Time
Model on spread of
disease.
Some Current Research
1. Over-use of antibiotics has
led to some bacteria
developing resistance. It’s a
big problem.
2. Scientists search for
antibiotics that can replace
current ones to which
bacteria have evolved
resistance.
In the old days,
scientists took soil
samples to find
fungi that killed
bacteria
One New Strategy
Forcing antibiotics to grow with
another kind of bacterium
might cause them to start
secreting an antibiotic to kill off
the competition.
Many bacteria have
genes that can
make products, like
toxins —even
antibiotics against
other bacteria.