Infectious Diseases
General Principles
Partnership for Environmental Education and Rural Health
peer.tamu.edu
Infectious Diseases are
Caused by Microbes
Vaccination can protect
against some infections
What is a Microbe?
• Microbes are microscopic organisms
that can live 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!
- as long as the temperature is right and there are enough nutrients-
Typical growth curve of an undisturbed population of microbes
… at normal temperatures (about 40 – 100 degrees F)
Can
You
Explain:
 Why growth becomes stationary?
 Why the microbes eventually die?
 Why this rapid growth does not occur at cold
temperatures? …at very hot temperatures?
Common Types of Microbes
Viruses
Bacteria
Fungi
Protozoa
What Do They Look Like?
Infectious agents look different, depending on what class of
agent they are
E. coli
Source: NASA
HIV Virus
Source: NLM.gov
Dutch Elm Fungus Campylobacter jejuni
Source: AGRIC.gov
Hepatitis C Virus
Source: Amgen, Inc; NIH.gov
Source: cfsan.fda.gov
Ciliated Protozoa
Source: blm.gov; Wilhelm Foissner
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,
commonly called
“germs” or “bugs.”
Bacteria
Bacteria can
look like balls,
rods, or spirals.
Source: NSF.gov
• Bacteria are unicellular
and prokaryotic
• They have their own
metabolism.
• There are countless
numbers of bacteria on
the Earth but less than
1% of them cause disease
in humans.
• Bacteria can live in a vast
range of places, but need
energy sources to thrive.
• Some bacteria produce
toxins that can harm us.
Definitions
• Unicellular - one-celled organism, self-contained
inside a membrane.
• Prokaryotic - organisms that lack a nucleus or
any membrane-bound organelles.
• Metabolism - chemical reactions that help living
things stay alive ... typically involve breaking
down compounds or building up structures or
secretions.
• Toxin - a chemical made by an organism that
makes another organism or animal sick.
Prions
• A prion is an infectious particle
made from an abnormally
folded protein found on the
surfaces of neurons.
• Prions are highly resistant to
heat, UV 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.
Viruses
• A virus is a microbe that
consists of a nucleic acid
housed within a protective
coat.
• The virus reproduces by
hijacking the host cell’s
metabolic machinery to
replicate its own DNA or
RNA.
• Most viruses cause disease
and are specific as to which
type of cell they will attack.
Source: niaid.nih.gov
Protozoa
• One celled microbes that
can be parasites or
predators of other
microbes.
• Can live in a variety of
places, but prefer moist
habitats.
• Usually cause disease in
humans.
• Protozoa are sometimes
helpful to other animals
(food source for whales
and help cows as well as
termites digest their food!
Protozoa found in human stool
samples
Source: CDC.gov
Fungi
• A multi-cellular microbe
that is much larger than
the other microbes.
• Only about 1/2 of all
fungi causes disease in
humans.
• Yeast is a fungus that is
used to make bread and
cheese for us!
Review Question #1
1. What is a microbe?
2. Name some kinds of microbes.
3. Describe a typical microbial growth
curve.
4. Why can some microbes live in hot
temperature, while most cannot?
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 and
Tapeworm
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.
Where Do Infectious Agents
Hide When Not Infecting You?
• Bacteria, protozoa,
and fungi all reside
in the soil.
• Other infectious
agents can live on
surfaces for hours or
even days, like the
cold virus.
Infectious Agent Life Cycle
• Once a few microbes enter the body,
it may take a few hours or days for
the agent to reproduce enough to
become infectious to others or to
cause disease.
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.
Review Question #2
1. How can microbes get in the body to
cause infection?
2. Name some diseases that illustrate
relative species specificity …. Name
some that do not.
3. What’s the difference between being
infected and being communicable?
What are the Main Types of
Infectious Disease?
Food Poisoning
Respiratory Diseases
Liver Diseases
Sexually Transmitted Diseases
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.
Respiratory Diseases
• Many infectious
agents attack the
respiratory tract.
• The most common
respiratory disease is
the cold, but others
include bronchitis,
influenza, SARS,
and pneumonia.
See our Web curriculum on respiration
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.
Liver Diseases
• Most liver diseases are
caused by a variety of
viruses. Best known are
hepatitis A, B, C.
• These viruses cause
hepatitis (inflammation of
the liver)
• You were probably
vaccinated for Hepatitis A
virus when you were young.
Source: consensus.nih.gov
A liver with varying stages of
hepatitis. The cells on the left
are normal liver cells.
Sexually Transmitted Diseases
• STD’s are a result of various viral and bacterial
infections that are transmitted through sexual
interaction or blood/serum exchange.
• There are more than 20 STD’s identified.
• The most common are:
Microbe
Disease
Chlamydia
Chlamydia trachomatis
Syphilis
Treponema pallidum
AIDS
HIV
Herpes
Herpes Simplex Virus
Acne is an Infectious Disease!
Acne is not
contagious, but it is
infectious
• The pimples are dead
white blood cells that
were attacking bacteria
in the skin’s pores.
• The sebaceous glands in
your skin make an oily
substance called sebum.
Too much sebum can
clog up the hair follicle.
This allows bacteria to
grow and multiply.
• White blood cells rush to
fight the infection.
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
Break the cycle of transmission
ill the infectious agent
ncrease host resistance
Review Question #3
1. List common types of infectious
disease.
2. Name a common infectious disease
that affects the intestines … the
lungs … the liver … sex organs …
the skin.
3. What are the three key steps for
dealing with infectious disease?
Activity Time
Demo 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.