What causes us to get sick? Many people answer by saying "germs." Actually, the "germs" that cause illness are pathogens—infectious agents that cause disease. There are four major types of pathogens: viruses, bacteria, fungi, and parasites (i.e., protozoa and worms). Pathogenic agents are highly specialized to infect body tissues and cause disease. 

Let's take a closer look at one type of pathogen—viruses. How does a virus attack? How does it get into your body? What happens once a virus enters your body? 

Directions: Watch Flu Attack! How A Virus Invades Your Body to see how the influenza (or the flu) virus can enter your body and cause illness.  

As you saw in Flu Attack! How a Virus Invades Your Body, each pathogen infects a specific type of cell in the body—its target cell. The flu virus specifically targets cells lining the respiratory tract (i.e., nose, mouth, and throat).  

Viruses identify their specific target cell by matching virus surface markers (keys) to cell receptor sites (locks). 

Your immune system is made up of dedicated cells, tissues, and organs that respond when a pathogen invades your body. Initially, responses involved in innate immunity (which are also called nonspecific defense mechanisms) respond immediately, so that an infecting pathogen can be quickly contained. Then cells involved in your adaptive, or acquired, immunity respond. These cells are highly specialized to target the specific invading pathogen.

Watch the following video on the immune system, paying careful attention to B-Cell and T-Cell.

HIV, which stands for human immunodeficiency virus, is the virus that causes AIDS. HIV targets and kills helper T-cells. Since HIV attacks cells that are critical to the body's immune response it effectively weakens a person's immune system, and makes them more susceptible to other infections.

Directions: Watch HIV Destroys Helper T-cells to see how HIV attacks T-cells and weakens the immune system.

Let's take a closer look at some of the illnesses caused by pathogens. As you read about each one, pay special attention to the pathogen that causes the illness—either virus or bacteria.

Directions: Click on each tab below to learn about the disease.

Treatment Options

There are a number of medicines that can help the body's immune system defeat invading pathogens. Antibiotics, for example, are medicines that fight bacterial infections. They either kill bacteria or stop them from reproducing, allowing the body’s natural defenses to eliminate the pathogens. Antibiotics often target structural components of a bacterial cell, like the cell wall.

Antibiotics will not work against viral infections like the cold or the flu. Symptom relief, like aspirin to relieve pain and fever, might be the best treatment option for viral infections. In some cases physicians can prescribe antiviral drugs, which fight viral infections by either stopping the virus from multiplying, or by strengthening the body’s immune response to the infection. Antiviral drugs are available to treat a number of viruses, including the flu virus and HIV.

Directions: Look over the Illness vs. Usual Cause table. For each illness, drag and drop the check mark into either the "Virus" or "Bacterium" box to identify the usual cause for each disease. Then drag the check mark into the "Antibiotic Needed" box if an antibiotic is an appropriate treatment for that illness.

Vaccines trigger the body's immune response to harmful viruses or bacteria so that the body can build up a resistance.

Directions: Watch How Do Vaccines Prevent Illness? to learn more about how vaccines work to immunize people against illness.

Overview

The Immune Response to an Invading Pathogen:

1. You are infected by a pathogen (e.g., the flu virus).
2. Through trial and error, your body figures out how to fight that specific type of pathogen.
3. Your body creates memory cells to be able to recognize and fight that same pathogen the next time you encounter it.

How Vaccines Work to Protect Us from an Invading Pathogen:

1. Vaccines help us develop immunity by imitating an infection. For example, a vaccine may contain inactivated virus particles that will not actually cause disease. 
2. Through trial and error, your body figures out how to fight the specific type of pathogen contained in the vaccine.
3. Your body creates memory cells to be able to fight that same (or similar) virus when you encounter it.
4. Your body will not need a trial and error period to mount an immune response upon exposure and will destroy the pathogen before it can cause disease.