Pick up almost any paper today and you'll see the shyly smiling face of Hannah Clark beaming back at you. (See the Sun, the Daily Mail and the Independent) And it's no wonder. Hannah's tale is that very rare thing: an unalloyed good news story.

Born with cardiomyopathy, a relatively rare condition which effects the muscle of the heart, Hannah's prognosis was poor to say the least. Her only real hope was for a heart transplant, and in 1995 aged just 2, that's what happened.

But as the Lancet reports in its online edition this week this was no ordinary heart transplant operation.

In a revolutionary new procedure nicknamed "piggy-backing", the team - lead by pioneering heart surgeon Sir Magdi Yacoub - grafted the donor organ onto Hannah's own heart, so that it took over much of the function of circulating blood around her body.

That gave Hannah's heart some respite, and the chance to slowly recover from its diseased state.

Ten years later Sir Magdi came out of retirement to oversee a second - reverse - transplant operation to remove the donated heart.

The problem with all transplant operations is the issue of immune rejection - the patient's own natural defences identifying the transplanted organ as alien, and attempting to destroy it. Hannah could have had a conventional heart transplant, but that would have condemned her to a lifetime on immuno-suppressant drugs and all the consequential illness that goes with it.

As it was Hannah was very ill for much of the time her donor heart was in place. She had problems with her lungs and spinal chord, and suffered from a particular form of cancer associated with transplant patients known as Epstein Barr Virus or EBV.

Given her age, Sir Magdi wanted to give Hannah the chance of a normal life. Implanting a donor heart to take the pressure off her own while it recovered, and then removing it at a later date, offered that possibility.

The benefits of that far-sighted decision were only too clear in the bashful, giggling - but healthy - 16-year-old who came into the studio this morning.

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Here's the graph showing the breakdown for swine flu cases by age that we were talking about on the programme this morning. It was put together by QSurveillance and used in the latest HPA report on the virus in the UK.

As you can see the normal bell curve distribution is strongly skewed to the younger end of the range, with 5 -14 year olds apparently worst affected.

There's still no clear explanation for why that might be. The latest research published in the journal Nature this week does show some striking similarities between swine flu and the virus responsible for the 1918 influenza pandemic.

That could explain why people born before 1920 have stronger immunity, but doesn't really account for the lower incidence of infection amongst those aged 45 and over. It could equally be that school classrooms and the playground simply offer a better environment for the transmission of infection.

One more point for the statisticians amongst you. The graph may inadvertently give a slightly false impression of the distribution by age because the bands are not all of the same duration. There were, for instance, 38 cases amongst children aged up to one year, but later columns cover longer periods.

If anything, the real bias towards the younger end of the spectrum is even more pronounced.

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The official figure for the number of people who've contracted swine flu stands at 6,538.

But it seems increasingly likely that this "official statistic" significantly underestimates the true scale of the problem.

The figures are compiled by the Health Protection Agency, and are based on the number of laboratory confirmed cases - that's people who have undergone a swab-test for the virus that has proved positive for infection.

But if you ring your GP today and try to make an appointment complaining of flu-like symptoms, the chances are you'll be told to stay at home, try and get your fever down, and get some rest.

In the vast majority of cases, where the infection is proving to be quite mild, that's good, sensible advice. It certainly helps to minimise the spread of infection around the doctor's surgery, but obviously if no test is conducted no report is forwarded to the Health Protection Agency and the case doesn't show up in the official statistics.

The result is that there's really no way to be sure exactly how many people have contracted the swine flu virus. Although it seems likely that the true scale of the epidemic is significantly higher than the official estimate.

So does that matter?

It should be stressed that in most cases a bout of swine flu seems to produce only mild illness - fever, fatigue, lack of appetite and coughing. In only a very small number of cases the symptoms have been more serious, requiring admission to hospital, and there have been three deaths across the UK.

Anyone experiencing more serious symptoms, including prolonged high fever, nausea and vomiting, or who may be in a high risk category or suffering from an underlying medical condition, is still being urged to seek medical attention.

But for most people it seems, a bout of swine flu may result in less serious illness than traditional seasonal flu.

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