THE TEEN IDOL left the secret townhouse in Columbia for the limousine that would take him to the Merriweather Post Pavilion less than a mile away. Despite security precautions, some of his junior-sized fans had already discovered the house and were rapping at the doors and windows like mosquitoes after blood. As the limousine approached the pavilion hordes of them appeared on the horizon, looking more like ants than mosquitoes now, swarming busily. Then the limousine was spotted and they became vultures, swooping down on the car with lusty appetities.

The barricades and guards were nothing to them as they batted against the long black car, reaching out with their little-girl hands, screaming as they saw their hero through the tinted glass. The car forged ahead to the backstage area, where the teen idol, a slim boy-man in a white-fringed suit, dashed into the safety of the theater and prepared to leap onstage and whip them into even greater frenzy. When he left after the concert, he ducked into an old station wagon and lay on the floor of it, the limousine a decoy that ran the gauntlet of screamers without him.

Fans. They aren't all so hysterical, but they are everywhere. You can see them in the vicinity of nearly any celebrity: sometimes piteously whinning, sometimes yapping like terriers, sometimes yelling like drill sergeants; pleading, expecting, demanding attention.

Sometimes they come in groups; sometimes alone. Usually their attention are benign, as they lavish their chosen hero with presents and compliments. But sometimes they are twisted, unguided missiles of dementia lost in the never-never land between fantasy and reality.

Mark David Chapman, the fan charged with shooting John Lennon hours after asking for an autograph, and John Warnock Hinckley, the man accused of shooting the president as part of an act of devotion for movie actress Jodie Foster, have given fans a bad name lately. A new movie, "The Fan," now showing at area theaters, continues the theme of fan as threat, prefaced with an announcement that points out it was filmed before the Lenon killing. UPI reported last week that Hinckley had checked the book "The Fan" out of the library in Colorado.

Fan behavior can be dangerous even when it isn't deadly. Stars -- from Nelson Eddy to Frank Sinatra to Elvis Presley and quantities of lesser lights -- have had their clothes ripped, their faces scratched, and their hair grabbed by fans. Fans themselves have died in pursuit of their adored one; a 14-year old girl, for example, died after being crushed at a David Cassidy concert in England; two fans committed suicide after Lennon's death. Fans are constantly being treated for hysteria and physical injuries, a catalogue of wounds suffered in an ongoing war of love.

But most fans are just having a love affair in their mind. "A long time ago Shakespeare said, 'What great man do, the less will prattle of,'" said Roland Hinz, editor of five fan magazines, including the 45-year-old "Modern Screen."

Within the family of fans are easily identifiable genera, species and subspecies:

Teen-age and younger girls: The sport, in this case, is often as important as the love object, who is usually an androgynous-looking television actor. The sport consists of screaming, jumping up and down, taking snapshots and swooning. Fevers are fueled by an industry of press agents, records, magazines and related memorabilia, all designed to rev the prepubescent motors up to high gear. A recent headline in "Tiger Beat": "Ralph's Lonely, But You Can Help! 'Eight Is Enough's' Ralph Macchio needs someone like you. Can you be there for him?"

Art fans, species Opera Ballet, Theater and Music. These fans are so legendary their bravos during performances are virtually part of the program. Many opera fans know the works as well as the performers; ballet fans will promote individual dancers' careers directly with management. Flowers are the usual symbol of worship. (At a recent performance at the Kennedy Center, for example, Renata Scotto was showered with posies; one fan heaved a bouquet from the first tier that the diva caught like a pro pass receiver.)

Necro-fans: Here, the love-object is dead. Species include devotees of Rudolph Valentino, James Dean, Elvis Presley, Marilyn Monroe. Known for annual pilgrimages to grave sites on the anniversary of the beloved's death.

Rock fans: These tend to be more prone to violent displays of attention, perhaps encouraged by the music. Signs of regard range from throwing things onto the stage to attacking a performer with a knife. A placid-looking Bob Dylan fan once started choking him, and then scarred his bodyguard, Mike Evans, by biting a chunk out of his wrist.

Autograph hunters: Again, sportsmen, with the trophy being the signature of a famous person. They tend to be harmless and usually say the autograph is for their daughter. Washington has a small group of "regulars" who can usually be sighted outside Channel Five during "Panorama," outside any major opening, personal appearance or sports event.

Groupies: A sub-species of Rock Fans and Autograph Hunters, referring generally to females whose only object is to have sexual relations with a rock musician or any member of his entourage. They tend to belong to geographical sub-sets rather than devote themselves to a particular singer or group. They have, however, been around since before the development of rock, as the history of paternity suits can attest.

Fan Club Members: A reflection of man's urge to organize. Always devoted to one performer. Sometimes cousins to:

Professionals: These are people who are basically fans but turn the energy of their devotion into careers for themselves. James Boswell, Samuel Johnson's biographer, is one of the notable examples of this phenomenon. Rona Barrett, the gossip columnist, is another. She writes in her autobiography that when she was 14, an overweight "cripple," she painfully hauled herself up countless stairways in order to take the subway from Queens into Manhattan to start and the run Eddie Fisher's fan club. The rest is history. Other professionals include paparazzi, celebrity secretaries and the occasional spouse.

Weirdos: Any of the above who go over the edge.

"It has been somewhat difficult at times," said Chen Sam, Elizabeth Taylor's personal assistant, who was still reeling from being smashed to the floor after the opening of "The Little Foxes" in New York. "People have pulled at her clothes, her hair, tried to pull her jewelry off . . . for safety reasons she has to have people around her to stop this sort of thing. I know it's enthusiasm . . ."

Taylor is one of the prime focuses of fan attention. Campaigning in Virginia for her husband, Sen. John Warner (R-Va.), she was often pursued -- at a distance -- by Elizabeth Taylor lookalikes. Most of them didn't look like her at all; they merely aped her dark hair and heavy eye makeup in fairly tawdry imitations. Some of them sauntered around with an air of confidence, almost as though they expected to be beseiged for autographs like their heroine.

Television stars seem to have more weirdos after them than movie or sports stars, perhaps because the familiarity of their presence in the living room is fertile ground for delusion.

"When I was a reporter in Chicago, there was a guy who became fixated on me," said Channel 7 anchorwoman Renee Poussaint. "He'd call two or three times a day, and he'd say that I was talking to him through the television, and that I was trying to contact him. And it got worse, to the point that he would fantasize actual encounters with me. He'd say that I was his girlfriend. He came around the station a few times. He would just hang around outside. He would always run away when I would come out and try to talk to him. He was investigated by the police; they found out he had been in prison, and had gotten a job at a parking lot five blocks from the station. He had a mental condition as well, and as a result they contacted his parole office and he was recommended for psychiatric treatment. Every now and then those things worry me."

And Channel 4 sportcaster George Michael remembers that while he was working for ABC in New York a woman approached him outside the building and pulled a gun out of her purse.

"Everything you've ever done wrong flashes before your eyes," Michael said. "The two guys I was with hit the pavement, and the security guard went to call the police and never came back. For 10 minutes I was pleading with this woman -- 'please lady you don't even know me.' Evidently she'd written me some letters and she said that I'd said some things on the air that were meant for her. Eventually she put the gun away. They arrested her, but I didn't press charges. It turned out she had been committed before. New York tends to breed these crazy people. Washington's a much friendlier town."

In India they call it "darshan" -- that special something you get from being in the presence of a holy or special person.

"It goes back to the idea of contagious magic," said Allen Dundes, an anthropologist who specializes in folklore at the Unversity of California at Berkeley. "You have to be in direct contact with the person, you have to actually see them. If you get the sign, you have a part of them, you have control." This may explain why some stars are told never to look a fan directly in the eye.

"There is tremendous power associated with these public, glamorized figures," said Michael Real, a professor in the telecommunications department at San Diego State College and the author of a book on "mass mediated culture." "A colleague wrote a paper on the Elvis Presley phenomena that included the story of a woman waiting around after a concert. She had actually touched Presley, or something, and described the experience the way Moses might describe the burning bush. It was almost a mystical experience."

Dundes compares the quest for autographs and other tangible pieces of stars to the trade in relics of Christian saints in the Middle Ages, or the Shroud of Turin. Those collectors, he says, "were Jesus fans."

"We'd prefer to be famous ourselves, but most of us are not," Dundes said. "Their success is our success. How can you belong? You have to try to get to them."

A peculiar thing about fans is that the object of their affections -- or distortions -- does not have to be world famous. A Washington columinst, for example, has received threats to his life and limb as well as the attentions of a regular fan who sends presents every holiday. Once he wrote about this fan and got a letter from her, pretending to be someone else, complaining that his fan was very angry. The return address was his own newspaper.

Channel 7's Lark McCarthy, who has a guard escort her to her car after work, got a letter from a man who wanted her to "dance naked with him in paradise." Once a child called her and said he was going to kill her, and once a man sent her an obscene letter complete with a $5 money order.

Stars themselves have an ambivalent attitude toward their fans. On the one hand they complain about the inconvenience of being recognized and pursued; on the other they get nervous if no one recognizes or pursues them. Many of them have invested much of their money to generate the kind of enthusiasm they later have to hire security guards (minimum cost: $250 a day) to control.

Once Sen. Gary Hart (D-Colo.) arranged to have dinner in Washington with his friend, Robert Redford. Redford asked that their table be located in the darker recesses of the restaurant so that he wouldn't be bothered by fans. As the dinner progressed and no one bothered them, one observer recalled, Reford seemed to be looking around expectantly. Finally a man approached the table. Redford prepared for the encounter. The man spoke -- but not to Redford. "Aren't you Gary Hart?" he said.

Many stars, though, do hate their fans, regarding them with a mixture of contempt and disdain. In some cases, it's largely because the "artist" believes the taste of the fans restricts him to mediocre material. For others the inconvenience of fame really does get to be an inescapable bore.

When Dustin Hoffman was making the movie "John and Mary" in New York, dozens of fans waited excitedly outside the bar where he was shooting pressing their noses against the glass, tapping and knocking like a swarm of bees. Finally Hoffman emerged for the sprint to a waiting limousine, the crowd held back by the security guards. Hoffman stopped and said loudly:

"Where were you when I had pimples?"