The symptoms include uncontrollable screaming, swooning and spending hours on Twitter and Facebook. It primarily affects preteen and teen girls, yet it is highly contagious and can infect mothers, too. In severe cases, sufferers camp out on sidewalks for days. "The appeal for me is, of course, that he's beautiful," says 15-year-old Emma Reeves of Madison, Conn., who has seen Justin Bieber twice in concert. "It's hard to find people who are successful, nice and care about other people and he has it all!"
By disease standards, "Bieber Fever" is approaching a global pandemic with the release of the 18-year-old pop star's latest album, "Believe," last week.
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Compared with past outbreaks of fan mania, scientists now have a better understanding of why teens—girls in particular—become so passionate about some musicians, and the recording industry is far more adept at exploiting the phenomenon. Parents of star-struck "Bieliebers"—as his fans are sometimes known—can be assured, experts say, that what looks like mass hysteria is a harmless stage in adolescent development. Long before the Beatles, Elvis and Frank Sinatra, frenzied female fans threw their clothing at 19th century pianist and composer Franz Liszt and fought over locks of his hair, say music historians.
Hearing familiar, favorite music stimulates the release of dopamine, the neurotransmitter involved in pleasure and addiction, providing the same rush as eating chocolate or that winning does for a compulsive gambler, says neuroscientist Daniel Levitin, who was able to observe the process using fMRI scans in his lab at McGill University in Montreal.
Dr. Levitin's research also showed that musical tastes formed in the teen years become part of the brain's internal wiring, as that is the time when some neural pathways are solidifying and others are being pruned away. That's why the music adults tend to be nostalgic for is the music from their teenage years.
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Boys also develop musical tastes in this phase of life, but adolescent girls are far more likely to become infatuated with pop stars, experts say, because they are awakening to romantic and sexual feelings that are both intoxicating and scary. Having a crush on a celebrity they are unlikely to meet is a way to try out such feelings at a safe distance. "A lot of girls I know practiced their first kiss on a poster. I don't think that's changed at all," says Mark Rubinfeld, professor and chair of sociology at Westminster College in Salt Lake City.
Boys are more likely to follow athletes intensely—partly out of a desire to emulate them and partly because rooting for a team conveys a sense of identity, psychologists say. Some of that carries into adulthood, as men paint themselves with team colors or skip work on game day.
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"Social critics worry about fans as hysterical, emotional girls and loner, stalker boys," says Joli Jensen, a professor of communications at the University of Tulsa, Okla. "Neither stereotype captures the experience of most fans—pleasurable, insider knowledge, playful imaginative connections about a public figure whose persona has particular appeal."
The music industry fuels girls' fantasies by promoting stars with sweet, boyish looks for as long as possible. "In my era, in the '60s and '70s, Tiger Beat and other teen music magazines would airbrush out the stubble on the teen idols," says Dr. Levitin, author of "This Is Your Brain on Music," who was a record producer before turning to neuroscience.
The songs are carefully calculated to play into young girls' romantic fantasies, from the Beatles' "I Want to Hold Your Hand" to Mr. Bieber's latest song "Boyfriend" with lyrics "If I was your boyfriend, I'd never let you go." (Representatives for Mr. Bieber didn't return requests for comment.)
"I've seen girls cry in the front row because they've touched his hand. I guess it's like tears of joy," says Isabelle Hunt, age 10, of Cranford, N.J., who heard Mr. Bieber at the Apollo Theater in New York last week.
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Social media has brought celebrities closer to fans, or it at least gives that illusion. "When I was younger, it was Shaun Cassidy, and we had to wait a week to see him on TV again," says Mathilde Forsell, a nurse in Westfield, N.Y., whose daughter, Carley Dugan, is a Bielieber. "Now, they can download videos 24 hours a day."
Mr. Bieber's 44 million fans on Facebook and 23 million followers on Twitter receive almost hourly updates on where he is and what he's thinking. On Sunday he tweeted love for his fans, and also this: "so the #Believe Zinepak is ONLY at Walmart. It has the deluxe CD and all the exclusive pics and content including interview and cards."
Fan fever has gone too far when children neglect school work, chores and real relationships in favor of fantasy love, experts say. "Too much is being on a computer for five or six hours a night following Justin Bieber blogs," says psychologist Alan Ravitz at the Child Mind Institute, a New York nonprofit research and treatment organization. Parents can go overboard in facilitating an infatuation, too. "A person who mortgages their house to go to New York and sleep on the sidewalk—that's an issue. But a parent taking time out of work to help their kid do something they really enjoy, that's OK," says psychologist Patrick Markey of Villanova University in Villanova, Pa. Some studies also show that children who go along with their peer groups tend to be the most well-adjusted, provided the activities aren't dangerous, Dr. Markey notes. "It may drive you crazy as parents to hear their music, but it's a sign of psychological health."
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Other experts say this sort of fan behavior isn't healthy. "Contrary to popular belief, there is nothing even slightly developmentally normal or healthy about 'Bieber Fever' and similar teen extremisms," says psychologist Robert Epstein, author of "Teen 2.0: Saving Our Children and Families from the Torment of Adolescence" and other books. He argues that parents should help their offspring recognize the hype and commercialization. "I'm all about fantasies for my 6- and 7-year-olds. But past a certain point, Santa Claus has to die," Dr. Epstein says.
Young music fans probably see through the fantasies more than parents realize. Lili Foggle, mother of two girls, ages 14 and 12, in Madison, Conn., says her daughters lost interest in Justin Bieber when he started dating actress and singer Selena Gomez. "The whole attraction was that he could really be your boyfriend. He's still selling that, but the older girls aren't buying it anymore," she says.
"I used to be, like, madly in love with Justin Bieber. Now I still like him, but I don't really love him, and I don't scream so much anymore," says Mrs. Forsell's daughter, Carley, age 11. She says she now prefers One Direction, a new boy band from Britain. Says Mrs. Forsell, "That's part of growing up too."