It’s very common for people to ignore the early warning signs of illness, and continue with physical activities, only to find that they make themselves quite ill. On the Dalmatian coast people usually swim in the sea from about April until October or November. Even in high summer, swimming with a sore throat, however slight, can lead to a bad chest infection. In the autumn, the risk is even higher, because the weather can be sunny and hot, yet very chilly in the shade or when it is windy. I’ve also heard doctors advise patients to continue going swimming while taking antibiotics for illness. One friend who took that advice when suffering from severe tonsillitis was ill for most of the next year with recurrent infections and ever-stronger antibiotics.

Occasionally in the run-up to an illness there is a deceptive feeling of unlimited energy and a need for strenuous physical activity. For example, a young man remembered feeling unusually active the day before he fell seriously ill with poliomyelitis as a child. He had spent several hours playing frenetically on a scooter. Afterwards the right leg with which he had been propelling the scooter was worst affected by the partial paralysis which weakened his torso and limbs. This false energy is a common effect in the onset of polio, and has been found to cause more severe degrees of paralysis.

I can remember feeling strangely energetic the day before I was taken seriously ill with pleurisy at the age of twelve. I went for a run on that wet, dark evening, something I had never done before, only to spend the next few weeks in bed, too weak to do anything except take medicines and read for short periods at a time. Twenty years later I repeated the mistake. While at the Winter Olympics as a physiotherapist to the British team, I spent a rare day off cross-country skiing enthusiastically for several hours, an unusual activity for me. I had been aware of a slight pain across my upper back that day, and had previously had a slight cold. After the skiing I realized I was seriously ill. Our consultant chest physician quickly diagnosed double pneumonia and pleurisy, and expertly guided me back to health over many months. In the many years which have passed since then, I have managed to avoid making that mistake for a third time, so far.

Exercising when ill can damage the heart. Certain sports, such as squash, are high risk for people who have high blood pressure or heart problems. Pericarditis is inflammation in the tissues covering the heart. It is a condition which can happen to very fit sports competitors who train or compete while they have an illness or infection, and it must be diagnosed and treated correctly to prevent further consequences. Exercising with infections can damage the heart muscle itself. Sudden death in sport is well-documented, but often fit young athletes think it will never happen to them, or they don’t know how to recognise the symptoms of illness which should be danger signs. I have known apparently fit sports players die while doing their sports: a 16-year-old junior international oarsman, a 21-year-old recreational squash player, and a County squash player of around 30. The oarsman had passed a stringent fitness test, including heart checks, just weeks before he died. The two squash players were known to have suffered from apparently minor infections  immediately before their deaths.

Overuse of antibiotics can contribute to chronic illness and so-called 'superbugs'. Doctors’ mistakes can play their part in sport-related deaths. Doctors dealing with sports participants have to have a lot of experience to be aware of all the possible problems which might affect a fit and healthy-looking individual. In the UK canoeists have for some years been issued with a fact-sheet giving guidelines for inexperienced doctors about Weil’s disease. The canoeists are advised to show them to any doctor they consult if they fall into potentially polluted waters. However, if the doctor ignores the guidelines, the canoeist is helpless unless a second opinion can be obtained quickly: I heard of a case a few years ago in which a canoeist died because the doctor refused to consider the possibility of Weil’s disease.

Fortunately, tragic outcomes from sport and energetic activities are rare. The better informed people are about how to deal with illness effectively, the better they can care for themselves, and the less they risk long-term or fatal complications.