4. Polio What it is: A viral disease that affects the muscles. How you get it: The polio virus lives in the intestines. You can get infected by coming into contact with a sick person’s feces. Why it’s serious: Most people get no symptoms or flu-like symptoms that last a few days, but polio can cause brain infection, paralysis, and death. It was one of the most feared and devastating diseases of the 20th century. Polio cases are down sharply thanks to vaccination, but the disease is not gone from the world.