But “Big Brother” has also achieved another, more distinguished milestone: this week it enters its fifth season–an astonishing run in the quick-burn world of reality TV. Despite all the attention focused on “Survivor” and “American Idol,” “Big Brother” is one of the most successful shows in all of television. With productions in 25 countries, it has been seen by more than 2 billion people. That’s billion with a “b.” Quite a following for a show that does little more than take a dozen contestants, lock them in a house for three months with 40 or so hidden cameras and watch the insane things they’ll do when they’re cut off from everything but the prospect of their own fame. People love this stuff. In Poland, one “houseguest,” as contestants are called, became so popular he was elected to Parliament after the show ended. “In ‘Survivor’ you see a lot of constructed situations,” says Mike Morley, an executive of Endemol International, the Dutch company that created “Big Brother,” “but the characters don’t come out. ‘Big Brother’ is more about the relationships. It’s like a living soap opera.”

In the United States, “Big Brother” averages 9 million viewers an episode, less than half the take of “Survivor” or “The Apprentice,” though it’s become a summertime staple for CBS. Since it’s on the air so much, “Big Brother” isn’t nearly as polished as other reality shows–it’s harder to craft a compelling story line when, as is often the case, you’ve got only 24 hours to do the editing. CBS has kept it around because its audience is one of the network’s youngest. When the show works, “Big Brother” is like a dorm on a perpetual all-nighter, where no one’s thinking too clearly and anything can happen. “For any good show, you want someone to root for and someone to root against,” says CBS executive vice president Kelly Kahl. “That’s the great thing about ‘Big Brother.’ These are people you hang out and make friends with. It’s like summer camp.” Let the games begin. Again.