Models are an important part of science; they help us see the world on a scale that works for us. In the first of five lessons on HIV, learners make a paper model of the HIV virus that is about 500,000 times larger than the actual virus. In addition to the model, the class discusses viruses as well as scale; it would take nearly two million HIV particles to fill one white blood cell. You may want to introduce the topic by reading aloud the "Portrait of a Killer" essay to set the stage for the lessons to come.