Aspirin, discovered in 1897, was probably most useful to the majority of people during the period

The answer that best fits the blank provided is "20TH CENTURY". It was during the 20th century that Aspirin was discovered, specifically, in the year 1897 by Felix Hoffmann, a German Chemist. For them, it was the most useful during this time in order to relieve pain.

The central idea is that the hiv/aids epidemic is serious.