Viruses cause many human (1), ranging from the common cold and the flu to more serious ones such as AIDS and COVID-19. On their (2), viruses are not alive. They become active when they infect or enter our bodies. They can do that easily because they are (3). Then, they start to act like living things and make copies of themselves. People have natural defenses (4) viruses. For example, a fever can make some viruses inactive. The body's (5) system also fights viruses. Vaccines cannot treat diseases caused by viruses, but they can help (6) the spread of disease. Vaccines contain viruses which are weakened organisms, and help the body's immune system make substances called (7), which fight diseases. If the same virus enters our body later on, the immune system will recognize it and know how to fight it off. Giving people vaccines can (8) a lot of lives. For example, two million people died of smallpox in 1967. (9), vaccines helped to stop this disease and made it disappear. Most recently, vaccines have played an important (10) in reducing deaths and severe illness caused by Covid-19 all over the world.