Backstory: Your sim or group of sims have a deadly disease! Though they show no symptoms, they are known as Asymptomatic carriers and can kill any sim they come in contact with. As a result, they have been ordered by the Sims Health Organization to be quarantined on an island for the rest of their lives. The sims can leave the lot on the island, but can never leave the island itself. They must survive on their own. The challenge ends when the last sim finally dies.
Here are the steps:
1. Make a Sim or Sim family. This can be as large as you'd like, and they can be any kind of occult, but they are not allowed to have anyone teenaged or younger in the family.
2. Move them onto one of the Isla Paradiso islands with an empty lot. Surround this lot with a chain-link fence and build some bunker-like buildings for them to live in.
3. This infected sim or sim family will not be able to return to the main islands for any reason. Nor can they visit any of the dive spots or sub-neighborhoods. They must grow their own food or fish for it. They can only earn money by using objects on the lot (Computers can be used to purchase real estate for instance). Sims are allowed to wander away from the lot to alleviate the "Stir-crazy" moodlet, but they cannot leave this island,
4. Kill any sim who happens to wander onto this island. You can use Objects/Delete if you'd like, or have a witch set them on fire, or even download the death note mod. Unfortunately, this means you will go through a lot of mailpeople.
5. These sims (if you have more than one) are not allowed to have any children.