Chapter 21

Diseases and the Body’s Defenses

21.1 Lesson 21.1: Infectious Diseases

Lesson Objectives

• List common causes of infectious diseases.


• Explain how the virus known as HIV causes AIDS.


• State how infectious diseases can be prevented.

Check Your Understanding

• What is a bacteria?


• What are the components of blood?

Introduction

Has this ever happened to you? A student sitting next to you in class has a cold. The other
student is coughing and sneezing, but you feel fine. Two days later, you come down with
a cold, too. Diseases like colds are contagious, or “catching.” Contagious diseases are also
called infectious diseases. Aninfectious diseaseis a disease that spreads from person to
person.

Causes of Infectious Diseases

Infectious diseases are caused by pathogens. Apathogenis a living thing or virus that
causes disease. Pathogens are commonly called “germs.” They can travel from one person to