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?

• 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.

What is a Microbe?

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

Fungi

Bacteria

Viruses*

ProtozoaPrion*

*See slide notes

Microbes and Disease

• Some microbes cause disease and some don’t.

• Microbes that cause disease are called infectious agents, we commonly call them “germs” or “bugs.”

Infectious agents are microbes that

can cause disease.

Bacteria• Bacteria are unicellular (one-celled) and

prokaryotic (they don’t have a nucleus).

• 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.

• Bacteria can live in a vast range of places, but need energy sources to thrive.

Bacteria can look like

spheres, rods, or spirals.

Source: NSF.gov

Let’s go over the types of Microbes:

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 TimeDiscuss:

What is the difference between living and non

living?

Protozoa• Protozoa are unicellular (one-

celled) 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).

• They are in the Domain Eukaryota and Kingdom Protista

• Usually cause disease in humans.

• Protozoa can be helpful to other animals

Protozoa found in human stool sample

Giardia

Yuck!

Fungi• A multi-cellular (many- celled)

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

• 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.

I can transmit Brucellosis

We can transmit lotsof infectious agents

including arena viruses and hantavirus.

I can transmitEbola virus!

Think about it:Where Do Infectious Agents Hide

When Not Infecting You?

The soilBodies of waterSurfaces like desks and tablesPeople’s skinIn the airOn 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 #21. 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

RespiratoryDiseases

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.

• White blood cells rush to fight the infection. The blood cells die and become pus.Acne is not

contagious, but it is infectious

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 TimeModel 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

Many bacteria have genes that can make products, like toxins —even antibiotics against other bacteria.

Forcing antibiotics to grow with another kind of bacterium might cause them to start secreting an antibiotic to kill off the competition.