Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: Twentieth-century research by such scientists as Jonas Salk on the use of vaccines has reduced the number of people who are infected with
polio, measles, mumps, rubella, and other diseases.