Must have either been from the final season, or a rerun. AIDS wasn't announced until 1981. Also... the March of Dimes (founded by US President Franklin D. Roosevelt, and named by Eddie Cantor) was started in 1938 to combat polio. After polio vaccine came along in the 1950s, they changed their focus, first from combating rubella (which can cause birth defects) to a full-time campaign against birth defects.

Lots of public service announcements can also indicate that commercial advertising slots weren't sold. CBSRMT's audience demographics skewed old, especially in the later seasons, hence lots of ads for items made to address various ailments. The ads you're hearing may be from a rerun years after the original airing, a rerun for which not a lot of advertising was sold.