Monthly resets of the viral achievement
Pretty self-explanatory. This wouldn't invalidate older viral achievements, just make new ones that would say something like "Viral Achievement: 2017 Jul". The players starting with the virus would be the top 8 people of all the official ladders (so best player in 1v1 ladder, seasonal, and realtime, best teams in 2v2 and 3v3. All the people in the #1 spot, in other words.). Also, perhaps the leader in coin revenue could also start with the viral achievement.
Other possible features that would go nicely with this, but are probably harder to code, so they're not absolutely needed:
Knowing the date on which somebody gets the virus.
Knowing from where you got the achievement (which game, from who)