In the movie Porco Rosso, it's mentioned that he has a curse which made him look like a pig. Why was he cursed to look like a pig and what was the cure, if there was one?
Should have looked harder before I asked the question.
According to Nausicaa.net's "Porco Rosso" FAQ, under the question "Why did Porco become a pig?"
Although it was never mentioned in the movie, a press release states that he was disillusioned with humanity, and cursed himself to be a pig. Miyazaki stated that "When a man becomes middle-aged, he becomes a pig". It seems that Porco is carrying a lot of baggage, and that has something to do with him quitting being a human.
The official story goes as follows: Captain Marco Pagot (named after the Italian animator and friend of Miyazaki - they worked together on Sherlock Hound for Italy's RAI TV) was an ace pilot of the Italian Air Force during World War I. He quit the IFA since he saw fascism on the rise, and he wanted to fly following his own will. He became a bounty hunter, assuming the name "Porco Rosso". He crossed out his own face as a young man in the picture that Gina had hung on the wall of her restaurant, so that no one would know what he looked like as a human.
Miyazaki said that Porco once intended to marry Gina, but then World War I broke out, and Gina was living on an island which was Austrian territory. As a military officer, he could not bring himself to marry an enemy national. Torn between his loyalty to his home country and his love for Gina, he chose his country. But when he witnessed the deaths of his fellow pilots, including that of his best friend (Gina's husband), he started wondering about the meaning of his actions, and the meaning of flying and dying for his country. Unable to resolve the conflicts in his mind, he became a pig.
Additionally, under the question, "Did Porco become a human again?"
The general consensus is, yes (though whether he stayed human is another matter).
In the last shot of the two of them wading towards their planes, Curtis was insisting on seeing Porco's face, implying something had changed about it. The conversation between Porco and Fio at the camp set up the "kiss the frog prince" theory, so Fio's kiss (and Gina's love) at the end did turn Porco back into a human. He became a pig because he was disillusioned with humanity, but Fio's innocence made him feel that "there is still some hope for humans", as Porco said. This time, he won, and he did not "make another girl unhappy". It seems that he finally overcame his self-hatred.
So I guess the answer's are "disillusioned with humanity" is what caused the curse, being that he cursed himself and that it's possible that he cured it at the end when he regained his faith.