There are currently more than 750 such areas in the National Wilderness Preservation System comprising over 109 million acres -- roughly 5% of the United States by area. Need a refresher from the other 95%? The Active Times picked 10 wild places in the lower 48 states that are truly wild.
Polio, which struck children disproportionately, could attack the central nervous system and cause muscle deterioration, paralysis and even death. American virologist Jonas Salk, who was born 100 years today on October 28, 1914, developed the worldâs first safe and effective polio vaccine.
The influenza virus was discovered in the early 1930s, and scientists developed a working vaccine by the 1940s, when it was first used on soldiers during World War II. In 1947, Jonas Salk, one of the vaccine's creators, began to develop a polio vaccine, which was perfected and approved in 1955. In 1976, an errant outbreak of swine flu at Fort Dix, New Jersey, caused scientists to worry about a possible pandemic, so President Gerald Ford announced that the federal government would vaccinate the entire U.S. population.