In less than a year, this epidemic has taken hold of tweenagers with symptoms including raving Tweeting, repeated viewing of YouTube, near-riots at airports and shopping centres and, in one isolated case, the stealing of a small boy's hat.
The source of the outbreak can be traced back to Canada - and who knew that big teeth, moppy hair and a voice high enough to make dogs wince three streets away could prove so devastating to the under-16s, whilst leaving grown-ups and contemptuous big brothers and sisters unaffected?
Justin Bieber, boy pop star, is currently the most talked-about person on Twitter, drowning out almost every other social media conversation. This week he also became the most searched for celebrity on the internet - even outstripping singer Lady Gaga, the publicity-seeking missile who combines the dress sense of Cher with the reticence of Brian Blessed.
Adoring Bieber more or less ensures future embarrassment for his fans, but right now the 5ft 3in Canadian is an object of desire who jets around the world, hangs out with President Obama and his family, and sang the opening lines on the celebrity-stuffed charity song for Haiti.
An appearance at a New York shopping mall caused such chaos that his manager is now facing a court case, and a flying visit to New Zealand prompted Bieber to Tweet reprovingly: "Finally got to New Zealand last night. The airport was crazy. Not happy that someone stole my hat and knocked down my mama. Come on people."
Musically, Bieber isn't that different from other teen idols; he sounds like Justin Timberlake before he left *NSync, or Hanson before they all left puberty. His songs are packed with mild hip-hop, spinclass-friendly beats, fizzy falsettos and squeaky clean gosh-I'd-like-to-take-you-out-for-lemonade-sometime lyrics. Songs such as One Less Lonely Girl, Love Me and Favourite Girl make it clear that Justin is aimed at a demographic for whom Hannah Montana seems a bit edgy and dark.
The difference is that unlike the Jonas Brothers, Miley Cyrus, Steps or David Cassidy, Justin Bieber wasn't propelled to fame by a popular TV show. He didn't even benefit from a tsunami of radio coverage. More than any singer, even Lily Allen or Susan Boyle, he is an internet phenomenon. Aged 13, he came second in a talent show and his mother Pattie put his song on YouTube. There he was starspotted by Scott Braun while scouting for someone else entirely, because he happened to click on a related link.
Braun used YouTube to build up Bieber's fanbase, feeding the site with artfully homemade-looking videos of Bieber singing to the camera, occasionally emphasising sincerity by thumping his chest or, accidentally, his stomach.
Some 85 million hits later, the baby-faced Canadian had both Justin Timberlake and R&B star Usher bidding to sign him to their labels. (Usher won.)
Bluesman Howlin' Wolf once sang: "What the men don't know, the little girls understand." Lucky for Justin Bieber, his management understands hero-worship. Instead of abandoning the internet, Bieber and Braun posted his progress on the web for all to track, from the incremental changes in hair to his new swaggering wardrobe. As well as YouTube, he has also been on Twitter for a year, although it wasn't until January, after the release of his first album My World, that his popularity put on a growth spurt. Now he adds another 11,000 new followers to his Twitter posts every day, and this accessibility has kept his fans hooked and reassured that their devotion is gratefully received.
"I love Justin Bieber," says a tearful three-year-old on a YouTube video that itself has turned viral "because I know he loves me back."
It's true that most of Bieber's Twitters are chirpy expressions of gratitude. "Show was AWESOME!!!" he tweeted this week from Denver. "Performing for all of u will never get old!! when U SMILE I SMILE!!" But as well as cheerleading his gang there is also an undercurrent of hard sell, with exhortations to help his songs reach number one. Bieber is single-handedly getting young people to buy CDs again, with two short albums, sold five months apart, shifting in their millions.
More than any other demographic, tween girls love mementos and keepsakes, and Bieber's special edition CD of World 2.0 includes software that allows the buyer to assemble a digital poster placing them beside their idol. Even the launch of World 2.0 was not on MTV or a major network, but the shopping channel QVC, home of Joan Rivers' imitation diamonds and dolls designed by Marie Osmond.
"It's cool to be launching my new album on QVC," said Bieber incorrectly, before adding more shrewdly: "Where else can I perform live and reach more than 98 million homes with my music?" What Bieber represents is pop marketing that has the illusion of the personal touch. But is Bieber really Tweeting? Manager Scott Braun says: "If he's not on his Twitter, I'll call him and say: 'Get on it.' As long as he keeps communicating with his fans, he can address any rumour directly because he's speaking to them."
There have been a lot of rumours to address - that he's dead, has syphilis, is having a sex change, and last week a hoax campaign claimed the singer had agreed to play a gig in the country most fans voted for. The winner was North Korea. Not everyone with a laptop is a Bieber fan.
The problem that faces every child star in showbusiness is growing up.
Bieber says that he wants a career like Michael Jackson's, by which he means performing music that everyone loves, rather than buying his way out of damaging court cases and dangling kids out of hotel windows.
While he may not miss what he calls a "regular" life - he has one day off a week and his mother, a bodyguard and a tutor travel with him on the road - Bieber still has regular teenage preoccupations. Officially he is "too busy" for girlfriends but couldn't help boasting: "I'm a great kisser. I've made out with a couple of chicks." More recently he admitted: "My mom said I wasn't allowed to date until I was 16, but I broke that rule. She found out and said: 'I'm disappointed in you.'"zz