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.