Infect is not a permanent counter on blocking creatures. Infect creatures simply attack that hits both power/toughness reduction in combat damage to the blocking creature. If a Wurmcoil Engine has blocked a 5/1 infect creature, Wurmcoil gets a -5/-5 power/toughness reduction UNTIL END OF TURN and Wurmcoil goes back to 6/6 at the next turn (if not hit by any spit damage while still 1/1). I'd rather PUT AN ENCHANT CREATURE or EQUIPMENT that has "put -X/-X on target creature" to keep the creature's power/toughness until equipment/enchantment stays on the creature.
Infect is not a permanent counter on blocking creatures. Infect creatures simply attack that hits both power/toughness reduction in combat damage to the blocking creature. If a Wurmcoil Engine has blocked a 5/1 infect creature, Wurmcoil gets a -5/-5 power/toughness reduction UNTIL END OF TURN and Wurmcoil goes back to 6/6 at the next turn (if not hit by any spit damage while still 1/1). I'd rather PUT AN ENCHANT CREATURE or EQUIPMENT that has "put -X/-X on target creature" to keep the creature's power/toughness until equipment/enchantment stays on the creature.