It might have been known to doctors, but it wasn't widely accepted yet by the public. Here's a quote from Wikipedia, about the retirement of Magic Johnson from the NBA after discovering he was HIV-positive. Note the dates:
"Nonetheless, Johnson was still voted into the 1992 All-Star Game, and the press speculated whether he was going to make a comeback. Moreover, many colleagues were wary: as HIV can be transmitted by blood contamination, they argued that Johnson would be a deadly risk if he suffered a bleeding wound while on court..."
Isn’t the “blood rule” still in effect for both the NBA and the NCAA?