If the user is Ghost Type and Curse successfully strikes the opponent, a parasitic bond will be formed between the two Pokémon. The cursed Pokémon loses 1/4th of its maximal HP per turn, while the user will immediately lose half of its maximal HP. The parasitic bond is only disformed when the Pokémon switches out or when the battle ends. Since this value is absolutized to the maximal amount, it would cause the user to faint if they presentially had only half of their maximal HP (rounded-down) or less.
The user's Speed will be downscaled by one stage; however, as a counterbalance, their Attack and Defense stats will also upscale by one stage.
The only preclusive counteragent against Curse is the Crafty Shield technique.