It can make grown men weep. Women become too terrified to look at the family photo album. Children start speaking in a foreign tongue.
What is this affliction? A weird, family-wrecking cult? A spooky medical syndrome? No, it's a voluntary, home based educational programme - a linguistic and cultural exchange called the Association for Learning Languages en Famille (ALLEF).
But this is no conventional week-long exchange. For parents pack off their eight to 11-year-olds to live with a French family for six months; the French child then comes to England for the same length of time. During the exchange, the only contact between parents and children is through telephone calls and letters.
The principle of ALLEF's exchanges is total immersion: children go to school with their French partner, and participate fully in the domestic and social life of the family and the community. Many English childrencall their partner's parents "Maman" and "Papa" and develop close relationships with the other children in the family. They absorb a different culture, a different way of life.
And, of course, they learn to speak French at a phenomenal rate - without a trace of that tell-tale British accent that either creases up or pleases the French so much. Fourteen-year-old Kim Percival, who went on her exchange when she was nine, delights in describing how she shocked French children she met on a campsite during a family holiday in France a couple of years ago. "We had been chatting away for days. Then one day, one of the girls came into our tent and heard me speaking English to my mum. She couldn't believe it. She went out and told everyone that I wasn't French. They couldn't believe it either. "
Kim, a Year 8 student from Middleton-on-the-Wold in East Yorkshire, recently took her GCSE French exams three years early - and got a grade A. But her success is not just academic. She looks back on her six months in the Ardennes as a happy time, unmarred by homesickness. Well, almost. "I missed my dogs a lot, but luckily my partner Cecile had dogs, too."
There were other strokes of luck during her exchange. "Me and Cecile are like twins," Kim says. "We're both quite tomboyish and played football and other sports with the boys, unlike the other girls at her school." They got on so well that since Cecile's return exchange with Kim's family, the pair have visited each other for fortnight-long holidays every year.
"Once when I went to visit her, we found that we had bought the same earrings and had cut our hair in the same style." Kim, Cecile and their families are friends for life.
Of course, it doesn't always work so well. Pam Owen's two sons, Nick and Joe, went on exchanges when they were 10, Nick to a village near Mayenne in northwest France and Joe to Valence d'Agen in the south. "The first child I had was Hugo, who was hyperactive. He'd go to bed at midnight and was up again at 6am. Nick had gone to Hugo's family first, and by the time it was Hugo's turn to come to us, in Middleton-on-the-Wolds in East Yorkshire, Nick was fed up with him. I didn't force them together, but luckily Hugo and Joe, my youngest son, got on well and he also made his own friends at school.
"In the end, no matter how much you plan, it either gels or it doesn't. But in this particular case, the parents weren't honest with us. It was a case of the perception you have of the other family and the image you'd like to have not being the same."
Pam Owen warns people who think that these long exchanges are a piece of gateau to think again. "It's very hard at both ends. The last time I saw my husband in tears was when we parted from our sons. And even when that's over, you have to be prepared for people coming up to you and saying, 'What kind of parent are you to send your child away for six months?' But for all that, the advantages they have gained in experience and confidence are tremendous. I don't regret any of it."
Over the past 20 years, ALLEF, a non-profit-making company run by parent volunteers, has organised more than 350 exchanges. It is careful to match children according to age and interests.
All participating families must produce school and personal references, and all are interviewed by national co-ordinators and local representatives in Britain and France, who oversee and support each exchange throughout the entire year. In addition, regional parents' support groups are set up to exchange information and to offer mutual support. Parents pay a registration fee of Pounds 500 to cover the costs of visits to France by the two national co-ordinators. It is up to parents to liaise with their children's school before the French child arrives.
Exchanges typically begin in late August or in February, so that the second leg coincides with a half-term break or a holiday. Families usually prefer to have a week's break between visits, to give the child who has been away time to settle in again before hosting the other child.
They also have to readjust to life in their own country, and their own language. Luke Shaw, 12, did his exchange when he was 10. "When I came back, I forgot how to speak English and I would speak French without realising it," he says. According to his dad, Charles, Luke spoke English with a French accent -"I even heard him sleep-talking in French."
Perhaps the most dubious accolade comes from Kim Percival's experience. "In France, I had to study English with everybody else," she says. "It was okay, but I'd forgotten a lot. So it was a bit strange when two children in my class got higher marks than me in our English test."