Imagine you're on a tropical island in the Caribbean. There are coconut trees, rocky cliffs, blue-green waters. But now, imagine there are hundreds of monkeys on this island. And, these monkeys have a disease that could kill you, if you're not careful. What you're picturing is a real-life island off the coast of Puerto Rico.

The island of Cayo Santiago hosts the oldest research center in the world for wild primates. Scientists from all over the world come to the island to study questions of primate behavior, cognition and ecology.

The island was created in the 1930s. Back then, biologists were spending a lot of time chasing monkeys around the jungles of Asia and Africa.

The idea, according to Richard Rawlins, the former director of research on Cayo Santiago, was to make things easier, by putting the monkeys on a small island, closer to the U.S., "where the animals were always there, could easily be accessed, and large amounts of data could be collected efficiently, within a short amount of time." The National Institutes of Health and the University of Puerto Rico fund the research.

Over the years, at least nine generations of rhesus macaques have yielded interesting insights into, among other things, kinship and the interplay between genetics and behavior — the sorts of questions that detailed, longitudinal research of a broad, free-ranging population can best answer.

It's sunrise, as I start my visit, and a bunch of scientists are climbing into a boat. They're wearing khaki short and boots. Some have backpacks. Someone fires up the engine and the boat begins to cross a calm channel of water. Up ahead, I can see the island — steep cliffs and lush green trees. The boat pulls up to the dock, and as we get out we scrub the soles of our shoes with a brush and dip them in pink disinfecting liquid.

We step onto the island. And right away, I hear this sort of spooky grunting noise. I can't tell where it's coming from. Then, up in a tree, I see two beige monkeys looking down.

"Just be careful they don't pee on you," says Giselle Caraballo-Cruz, a member of a team of researchers from the University of Puerto Rico who manage this island, which is called Cayo Santiago. Caraballo-Cruz actually has one of the best job titles ever — monkey island census taker.

But her warning about not getting peed on is serious. These monkeys naturally carry herpes B, a version of the virus that can be deadly to humans. So rule number one on Cayo Santiago: Don't get peed on by the monkeys.

We walk along a dirt path, toward a large metal cage. It's where the scientists eat lunch. At Cayo Santiago the people are in cages and the monkeys are free, Caraballo-Cruz explains.

"It's like a zoo where the monkeys come to see the humans," she says. "They can see us eating."

Rule number two: The monkeys are the free ones.

But it's not time for the humans to eat just yet, so we keep walking — up a steep hill. Caraballo-Cruz spots a small monkey off the side of the trail, and suddenly veers way to the left. She gestures at me to follow. Steer clear of all baby monkeys, she advises, or risk a run-in with protective mother.

But if you see an adult male monkey, Caraballo-Cruz says, don't be intimidated — because the males like to mess with people. Recently, she says, a college student on the research team let a monkey bully her.

"She would run away," Caraballo-Cruz says, "and the monkey would, like, chase her." Eventually, the student stopped running, and the male macaque stopped picking on her.

And that's rule number three of Cayo Santiago: Stay away from the baby monkeys, but show the adult males who's boss.

We get to the top of the hill and the view is spectacular — the mainland of Puerto Rico behind us, open ocean in front. A white pickup truck drives past, and some men get out and unload big white bags of monkey chow. It looks like dog food. And suddenly, there are monkeys everywhere.