A health official administers polio vaccine to a child in a poor neighborhood in Lagos, Nigeria, in 2004. Polio is a highly infectious disease caused by a virus. It invades the nervous system and can cause total paralysis in a matter of hours. It can strike at any age, but affects mainly children under age 3. Today, the disease has been eliminated from most of the world, and only seven countries worldwide remain polio endemic. At the same time, areas of transmission are more concentrated than ever -- 98 percent of all global cases are found in India, Nigeria and Pakistan. (© AP Images)