Swamp Sickness

When Swamp Sickness comes into play, put a
sickness counter on up to three different target
creatures.
Whenever a creature with a sickness counter on
it is blocked, the blocking creature gets a sickness
counter if it doesn't already have one.
At the end of each player's turn, that player
puts a -1/-1 counter on each creature they control
with a sickness counter on it.