Students learn how viruses invade host cells and hijack their cell-reproduction mechanisms in order to make new viruses, which can in turn attack additional host cells. Students also learn how the immune system responds to viral invasions, eventually defeating the viruses—if all goes well. Finally, they consider the special case of HIV, in which the virus' host cell is a key component of the immune system itself, thereby severely crippling the immune system and ultimately leading to AIDS. Note the recommended order to conduct this lesson/activity set: After presenting the lesson's Introduction/Motivation content, conduct the associated activity—a dramatic hands-on simulation that illustrates how quickly a virus can spread through a population, and then challenges students to determine who the initial bearers of the virus were. Then return to the lesson, with students ready to learn more about viruses.