Destroy target creature, then proliferate. (You choose any number of permanents and/or players with counters on them, then give each another counter of a kind already there.)
6/1/2011 If the creature is an illegal target when Spread the Sickness tries to resolve, the spell doesn’t resolve. You won’t proliferate.
7/1/2013 If the creature regenerates or has indestructible when Spread the Sickness resolves, you’ll still proliferate.