Answer:
Vaccines
Explanation:
In the early 1950s, before polio vaccines were available, polio outbreaks caused more than 15,000 cases of paralysis each year. Following introduction of vaccines—specifically, trivalent inactivated poliovirus vaccine (IPV) in 1955 and trivalent oral poliovirus vaccine (OPV) in 1963—the number of polio cases fell rapidly to less than 100 in the 1960s and fewer than 10 in the 1970s.
