Lust for life: Cystic fibrosis sufferer Alex Stobbs on conducting a three-hour Easter epic by Bach

You might already know a little about 19-year-old Alex Stobbs thanks to the Channel 4 documentary A Boy Called Alex broadcast last year. Few who watched the film were not haunted by the sight of Stobbs, as frail as he was determined, overseeing a performance of one of the great compositions of the classical canon. But what happened next?

Stobbs was born with cystic fibrosis, a lung-wasting disease whose suffererers' average lifespan is 31 years; it's slowly destroying not only his lungs, but also his bones and his digestive system; the drugs he must take have made him partially deaf. He lives under the constant threat of a fatal infection, and of a sudden catastrophic loss of blood. His doctor describes this latter threat as a timebomb – and in the middle of the documentary that timebomb went off. ' Stobbs' nurse found him lying in a bed full of blood: he had haemorrhaged at two in the morning, but hadn't wanted to wake people up, so he stoically waited for the nurse's arrival at dawn. Stobbs survived the trauma by a whisker.

But that's only half the story: the other half is that Stobbs is a brilliant musician – a virtuoso pianist who is now emerging as an accomplished conductor. The film's framing drama was his creation from scratch of a polished performance of Bach's Magnificat, directing 60 fellow pupils at Eton. When almost at death's door, he demanded that his doctors let him out of hospital for a key rehearsal; everyone agreed that the healing effect of pursuing his dream outweighed (just) the calamitous effect the excitement might have on his health. If his parents and siblings were devoted, so was everybody else: his friends, it was clear, would die for him.

"Are you ever afraid?" asked an interviewer at one point in the film, to which he wryly replied: "No. In order to enjoy stuff, you can't be scared." As people milled admiringly round him after the performance, he was asked about his next musical plan, and answered quick as a flash: "Bach's St Matthew Passion – all three hours of it." It sounded like an off-the-cuff joke, and may even have been one, but next Sunday that is what he will conduct – with the Southbank Sinfonia and the elite Rodolfus Choir plus top-flight soloists – at an already sold-out performance in London's Cadogan Hall.

Now a King's Scholar at Cambridge, Stobbs agrees to meet me for lunch, on a busy day between lectures and filming a scene for a sequel to the original documentary, to be broadcast in the autumn. Stobbs breezes into the restaurant, flings down his coat, and avidly studies the menu before settling for steak and chips. His handshake feels alarmingly fragile, but he's hyper-alert, and he radiates cheerful force.

The lecture he's just attended was on early sacred music, and he's enthused. He doesn't go a bundle on music theory: what gets him going is harmony and counterpoint, understanding how music works – and playing it on the piano, most particularly big concert pieces by Chopin. He started the piano at three, and has been winning prizes ever since; if illness had not weakened him – and playing the piano is a strenuous activity – he could have been a contender as a concert pianist. He has always composed, and always sung. Life really took off for him when he joined King's College Choir. "What I loved was the perfection they demanded – to get everything right every time. They're all so wired about getting anything wrong – and that attitude is a great thing to have. And it's fantastic at that age [12] to sing at the Sydney Opera House and the Concertgebouw in Amsterdam – it sets you up for life."

His first shot at conducting came when he was 14. "I asked to be allowed to conduct a double-choir motet by William Harris with the choir at Eton," he says. His speech is fast, and slightly slurred. "It was quite complex to conduct, but it went OK. Then I had a bad patch and wasn't so well and had to go to hospital, but I'd been preparing the Bach Magnificat for over a year, and it still went well. Having been in a choir, I knew what was needed of a conductor."

For "wasn't so well" read "nearly died", but the way he tells it makes the trauma sound almost routine. He gets a letter out of his pocket, then puts it quickly back without showing me. It's from a leading conductor whose advice he has sought with regard to his impending performance of the Passion, but tantalisingly he can't decipher the handwriting: "It's so important that I'm going to take time to read it properly. But I've also got five recordings of the work on my iPod." The one he likes best is the fastest.

Any normally fit person would be challenged by what Stobbs has set himself to do this month, including a regional tour with the King's Scholars, followed by a frantic five-day run-up to the Cadogan Hall event. "I'll have to be on my best behaviour," he says, by which he means he can't afford a health crisis. Immediately after this interview, he's going to have an Alexander-technique session for posture and breathing: beneficial for everything, he says, adding that he's even got taller as a result. "The best thing that's come out of this year so far is that I'm singing so much – I haven't missed a service."

Two hours' singing a day (he's a tenor) is a fine way to combat the mucus which builds up in his lungs, and which has to be constantly beaten out of them, to obviate the danger of a fatal infection. "And now I'm over 18," he adds jubilantly, "I have the right to decide what medication I have." He has temporarily refused a two-week course of additional intravenous treatment (which would make him drowsy), but ruefully accepts it would be advisable. His daily routine involves several infusions of oxygen and several bouts of physiotherapy, a variety of injections and 60 pills. One of his brothers (he has two, and a sister), living on the floor above at his Cambridge college, helps him consume everything as prescribed; an electric scooter helps him rush from one appointment to another. He never evades a question, but constantly steers our conversation away from illness and back to music; periodically he's convulsed with a terrible rattling cough, but not even that impedes the eager flow of his ideas. I remember his mother's chilling aside in the documentary: "Time's running out."

Proceeds from the Cadogan Hall concert will be given to cystic fibrosis research, as will royalties from the autobiography Stobbs is currently writing for Hodder. The disease doesn't have a high profile, and he wants to remedy that. "The main area they are looking into is gene therapy," he explains. "But I don't like to think about the disease itself – I'm not interested in keeping up with the latest developments." A pause. "I find that... just not very happy. Obviously I'm glad if they can reduce its effects, but a cure is over there somewhere" – and he gestures into the distance. "Of course, I'd love anything to reduce the strain on, my mum mainly."

I say how struck I am by how normal he seems to be, given these abnormal stresses. "Yeah, I try to keep it in the background – try not make a big deal of it. In the school playground you want to be as like the others as possible, and cricket's been good in that respect – I've always just about maintained my playing at my peers' standard." In his early teens, he was an outstandingly good batsman. "The first service I sang in after my hospitalisation last autumn, I could hardly walk down the aisle, my lungs were so bad. But I've now done two terms here without a major incident."

Does he get depressed? "I get down sometimes, but it's always about my work or my social life – it's never about cystic fibrosis. That's an afterthought. I'm basically really happy." And full of appetite, including for things like Twix bars. "I need three times the amount of calories other people need in a day, so this steak and chips is ideal. I also have an overnight drip-feed, which gives me another 2,000 calories. If I don't have that, I really feel the difference when I wake up. I burn up calories much faster than other people do."

As it's time for the Alexander-technique session, our meal ends abruptly with Stobbs speeding off into the traffic on his electric scooter, having invited me to pose any further questions by email. Later, I talk to his mentor Ralph Allwood, director of music at Eton, who puts it all very neatly: "He teaches the rest of us how absurd self-pity is. And he raises our spirits, because his own are almost always so high. And, wedged as we are between two eternities of idleness, he reminds us that there's no point in being idle now."

His mother Suzanne, who has doggedly nursed him through the bad times and gives thanks for the good ones, tells me: "He always puts on a front, but he's not as well as he says he is. He gets very tired, he pushes himself too hard, and his lung capacity is not as good as he would like to think it is. He's got only a third, and when he's ill, it can go down to 28 per cent. He's chronically ill, and it's amazing what he's done. But he's lost weight recently. He has osteoporosis, and liver disease, too. But he lives for the moment, and he loves having a focus."

Then she comes out with a clinching truth. "He's in denial, even though he knows exactly what is going on. But the denial is his way of coping."

Alex Stobbs conducts Bach's 'St Matthew Passion': Cadogan Hall, London SW1 (020 7730 4500) on Sunday 5 April. A version of this article appears in the current issue of 'Classical Music' magazine

Bach's Easter marathon: The challenge of 'The Passion'

As The Messiah crystallises our feelings for Christmas, so the monumental St Matthew Passion, written in 1727, gives definitive expression to the historical events celebrated at Easter. It is the most complex of Bach's vocal works, a cyclical pattern of narrative, comment, aria (prayer) and chorale. Alex Stobbs' intention to memorise all three hours of it is impressive. The opening fantasia alone is a major challenge: it employs two separate choirs, each with its own sub-orchestra. There are two organs, and the soloists are divided into parallel groups, and are in constant dialogue with each other and with the main chorus. Dauntingly, there are few dynamic markings (indicating how loudly the music should be played): everything must be conceived by the conductor as part of the huge ongoing dialogue. Alex will be able to sit down during the arias and recitatives, but there will only be one interval in this gruelling test of stamina. MC