Every week in the UK, 12 seemingly fit and healthy people aged 35 and under die from a previously undiagnosed heart condition.

They often bear no symptoms and are at the peak of their lives. But the simple act of walking into a cold shower, hearing the doorbell ring or playing sport can be enough of a shock to the heart to kill them.

The post-mortem examination of Boyzone singer Stephen Gately, who died aged 33 earlier this month, revealed he died from a build-up of fluid on his lungs known as acute pulmonary oedema. This obscure cause of death is often linked to an underlying heart complaint, and Stephen’s family believe he died from a previously undetected, hereditary heart condition.

According to the charity Cardiac Risk in the Young (CRY), many young people have no knowledge that they’re at risk from heart problems.

The charity estimates that sudden cardiac death kills more than 600 people aged 35 and under a year – although experts believe the number is higher as many sudden death cases are wrongly recorded as asthma, epilepsy or drowning.

Stephen was the most recent high-profile victim of what is known as sudden cardiac death, an umbrella term for a number of heart conditions affecting fit, healthy people, says Dr Sanjay Sharma, consultant cardiologist at King’s College London.

“The most common cause in older people is coronary artery disease, usually due to smoking or high cholesterol and causes blocked arteries in the heart,” he says. “In young people, however, sudden cardiac death is due to a structural problem or an electrical fault in the heart and is usually inherited from heart conditions in families.”

Hypertrophic Cardiomyopathy, a so-called structural problem, is the most common cause of sudden death in young people, affecting 10,000 people in the UK, according to CRY. It is caused by abnormalities making the heart muscle become thick and render it vulnerable to potentially fatal rhythms.

Whatever the heart condition, young victims usually have no symptoms. Even when there are symptoms in a young person, they’re often disregarded by their GP, says Dr Sharma.

Sudden cardiac death is most common in adolescents and young adults when the body is most sensitive to adrenaline. The condition is thought to be stressed by a sudden surge of adrenaline, which the body can’t cope with.

Dr Sharma says athletes are at the highest risk of suffering from sudden cardiac death, as “exercise can cause rhythm imbalances in people harbouring these faults in the heart”.

Delving into your family’s health history is key to understanding your own risk, and a medical examination can help find any inheritable structural heart disease.

But further tests, such as an ECG (electrocardiogram), which takes electrical readings of the heart, echocardiograms (which use ultrasound to look at the heart structure), and exercise tests, can also help determine a heart condition.