Kay Underwood, 20, has cataplexy, which means that almost any sort of strong emotion triggers a dramatic weakening of her muscles.
Exhilaration, anger, fear, surprise, awe and even embarrassment can also cause sufferers to suddenly collapse on the spot.
Victims are often left paralysed for several minutes, although they always retain the ability to hear what is going on around them.
Kay, of Barrow-upon-Soar, Leicestershire, who was diagnosed with the condition five years ago, once collapsed more than 40 times in a single day.
She said: "People find it very odd when it happens, and it isn't always easy to cope with strangers' reactions.
"Once, when I collapsed on some stairs, a woman walked past, hit me over the head and said I should have collapsed in a more convenient place.
"But I've learnt to live with it. I can tell when it's going to happen and have learnt to fall in a comfortable position or find something to lean on."
Like most cataplexy sufferers, Ms Underwood is also battling narcolepsy - a condition that makes her drop off to sleep without warning.
Narcolepsy affects around 30,000 people in the UK and about 70 per cent of them also have cataplexy.
Dad's Army star Arthur Lowe was cruelly afflicted by the problem in later life and once famously dozed off during a live TV interview.
There is no cure for either condition and theories on the causes range from brain chemical inbalances due to an immune problem or as the result of an infection.
Ms Underwood is now hoping new medicine can give her a normal life after visiting a specialist in sleep disorders at Leicester General Hospital.
Dr Andrew Hall, who treats around 200 narcolepsy sufferers, said science had still to determine the exact cause of both conditions.
He added: "Cataplexy affects people in different ways. It's not just laughter than can make them collapse - it's usually extreme emotion.
"I had one patient it happened to whenever he had an overwhelming feeling of smugness. But the causes aren't really known."