There are many different versions of the story behind the Chinese and Japanese zodiac. The version used in the manga is the one where GOD told all the animals to come to a very important party. The mouse decided to decieve the cat by telling him that the party was at a later date, so while the other animals were climbing up a hill to the party, the cat was at home sleeping. The placement of the animals in the zodiac apparently depended on when they got to the party, so the first animal was the mouse who rode in on the Ox's back. Then the tiger, rabbit. dragon, snake, horse, sheep, monkey, rooster, dog and boar.
Another version of this story is the following:
This is an invitation to all animals.
Whoever arrives at my place on the first day
of the year, at sun rise, will become my guests of honor.
The first twelve will be chosen as leaders
to whom l will grant the whole year to look after.
And from that moment on, the years will forever be
called after your names.
Then the Master went to the top of the mountain, and from there scattered
the letters that were soon carried by the wind to the land below. The animals were all very excited to get the letter, and most of them were very eager to take part in the race. The cat, who had been sleeping all day long, was too lazy to read the letter, and so he asked a rat who happened to be there, what it was all about.
Meanwhile, the cow, who knew that he was slow, decided to take off at midnight when everybody was still asleep. He took care not to make too much noise, but the clever rat knew all about his plan. He climbed and sat silently on the back of the cow, feeling quite happy and comfortable there. On the day of the race, the animals woke up very early in the morning before sunrise. Frogs, elephants, monkeys, bears, giraffes, dogs, tigers, roosters, snakes, all started to run to the direction of the Master's house. There were fights between the dogs and the monkeys who blocked each other's way, snakes hissed at each other, tigers howled, and everybody ran with all their might accross the valleys and the rivers. Soon the sun began to appear on the horizon. The cow, who was at the head of the race, was very excited when he saw that the house of the Master was only a few feet away, "I'm the winner!" he shouted happily, heading straight to the Master who was standing there waiting for the animals.
"Happy New Year, Master!," he said, when suddenly there was a shrill shout. The cow was surprised to see that a rat was right there in front of him, greeting the Master proudly. The rat had been riding on the cow's back all the way, and jumped down just at the right moment to be the first one to arrive at the Master's house! So it was the rat that became the first leader of the year, followed by the cow, tiger, rabbit, dragon, snake, horse, sheep, monkey, rooster, dog, and boar. The others animals who lost the race went back to their dwellings, while the 12 winners became the Master's guests of honour. The Master entertained them with all sort of delicacies and drinks, and named the coming years after their names, begining with the year of the rat. Suddenly there was a terrible noise. A cat came in with a loud cry and furious eyes. He jumped in and started chasing the rat, swearing and shrieking that he was going to kill him for cheating about the race's date. The cat was so angry that he was not included at the naming of the year, that even today cats never fail to chase rats whenever they see them.
The curse's victims are random. In the Fruits Basket Character Book, Takaya Natsuki actually comments that it doesn't matter if your dad, brother, etc. was cursed or if you're an inner or outer Souma, the selection process is completely random. It does seem, however, that there are only 13 cursed Soumas at any given time (i.e. two Soumas would never be cursed by the tiger at the same time) and that hundreds of generations of Souma's have been affected by it. What is unclear is whether or not those cursed by a particular animal are of the same gender. So, let's take Kisa as an example here, was the person cursed before her a guy or are all the Souma's cursed by the tiger the same gender - so all female? Because it's all random, your gender probably doesn't really matter, but it's still an unresolved question.
Also, when one cursed Souma dies, how long does it take for the next cursed Souma to be picked? For this question, let's look at Kyou. We know that Kazuma's grandfather was cursed by the cat (Kazuma is Kyou's gaurdian. For more info look at Kyou's "more info" section), and I suppose that it's possible that he was the cursed cat Souma right before Kyou, but the problem is we don't know *when* he died. Maybe he died and for a while no one was born cursed by the cat until Kyou came along. Or maybe someone else was born cursed by the cat, but that child died young. Of course, if Kazuma's grandfather died right before Kyou was born then that would answer this question - that is, that the curse is successive in that when one "host" dies it moves on immediately to another host.
It also seems that of the cursed 13, there is usually one member who has the power to erase people's memories - as a defense mechanism if someone outside of the family finds out about the curse. In the manga, Hatori has this power. It's not clear whether or not the Souma's cursed by the dragon are the only members able to possess this power. And it's not clear if one Souma member, at any given time, has this power or if it skips a generation.
One other related detail regarding the curse is that the juunishi plus the cat seem to obey exclusively the wishes of the head of the family. In the manga, this person is Akito. Akito is apparently seen as a "God" by the juunishi (these are Akito's words by the way, Ch. 61), but we don't know from what line he descends from and if his father or mother before him held this position. Perhaps being the head of the Souma family is also a curse and so the selection process here is also random? We don't really know what their role is in the family and in the curse itself. Their function seems to be one in which they monitor the cursed 13 and prevent the rest of the family and the outside world from finding out about this curse. Is Akito just a figurehead who watches over the cursed 13? Or maybe there's someone else in the family who's even stronger than him?
In general, we really don't know all that much about the Juunishi Curse. So far, Kazuma's grandfather is the only prior cursed Souma that we've seen in the manga, so we really don't know all the details of how the curse works exactly. What purpose does it have? There must be some reason why it exists and why the Souma's seem to be the only ones affected by it. In my "Souma Family" section, I talk about the wealth of the Souma's and I theorize that the curse may actually be related to the family's wealth and success. Maybe the cursed 13 take on the burden of the curse so that the rest of the family can be successful? ...When more information is available, I'll definitely update this page. If you have any theories of your own, don't hesitate to email me ^_^.
1. The cursed 13 turn into their respective animal of the Japanese zodiac when hugged by someone of the opposite sex. If hugged by someone of the same sex or by another cursed Souma (male OR female), nothing happens. Among the juunishi, it's common knowledge that falling in love and marrying someone who is also cursed is an ideal match...
2. When the transformation occurs there's a puff of smoke (in the anime and manga) and there's no way to gauge how long the transformation will last. If someone of the opposite sex continues to hug one of the 13 while there are in animal form, they will remain in that form (for ex. when Tooru hugged Haru's neck as they were trying to get Yuki back to Shigure's house).
3. When they return from their animal forms, they're completely naked! ^_^
4. Many of the cursed 13 also seem to have health problems as a result of the curse. These health problems are associated with the weather or their general mood and when they get very sick (a cold or something) it's very common for them to suddenly transform into their juunishi forms.
5. The cursed 13 also have the ability to attract other animals depending upon their mood. So, Yuki attracts mice, Kyou attracts cats, etc. They can call upon these animals to help them out when they're in trouble. So in vol. 1, when a landslide covers up Tooru's tent, Yuki calls his 'lil mice buddies to help him uncover and retrieve her stuff. It also seems that cats are attracted to Kyou when he's angry or depressed ^_^.
6. They also take on many of the same physical attributes as the animal they are cursed by. So, for example, Kyou has the ability to jump out of two - story buildings and not get hurt (just like a cat) and Ritsu can climb onto the roof of Shigure's house, almost fall off of it, and scramble back on (like a monkey ^_^;). The juunishi also resemble the animals that they are cursed by, hence Yuki's gray hair, Kyou's reddish/orange hair, Haru's white and black hair, etc.
7. There are never two juunishi cursed by the same animal at the same time. It seems as though there is always one juunishi for each animal at any given time. So, for example, we haven't seen two Soumas cursed by the tiger. There's just one right now - Kisa. And Momiji said as much in vol. 10 when Hiro finds out that his mom is going to have a baby. Momiji says something to the effect that he's glad that this baby will be "safe" because all the juunishi have already been born.