The ability to sit at a piano and croon with an accent like Charles Aznavour's is for many the acme of seductiveness. And people will pay to ensure their French accent is as perfect as their command of the Gallic idiom. But what if you woke one morning to find your usual Liverpudlian tones supplanted by a heavy French accent?

For those with a little-known neurological condition called Foreign Accent Syndrome, that question is not an idle consideration but a day-to-day reality. Kay Russell, a 49-year-old from Gloucestershire found herself subject to a media maelstrom last week when it was reported that she woke from an afternoon nap to find her clipped Home Counties accent supplanted by a distinctly Gallic lilt. Having endured a series of hemiplegic migraines that caused minor brain damage, it is highly unlikely that her former accent will return.

Foreign Accent Syndrome is not the only strange condition to affect people. According to Rare Disease UK, a support network for sufferers, there are an estimated 3.5 million people with a unusual illnesses in Britain today, ranging from a phobia of bridges to hallucinations of gigantism.

Foreign Accent Syndrome

This little-known condition alters the intonation of the sufferer's speech, giving the impression that they are speaking with a foreign accent. Arising from damage in the left frontal lobe of the brain, the condition is often found in stroke victims and is often a transitory in nature.

Other sufferers have included the Newcastle woman who developed a Jamaican accent following a stroke, and the migraine sufferer from the West Country who woke up one day with a distinct Chinese accent.

As Professor Nick Miller of Newcastle University points out, though, the syndrome has less to do with foreign accents than the name implies. "The notion that sufferers speak in a foreign language is something that is in the ear of the listener, rather than the mouth of the speaker. It is simply that the rhythm and pronunciation of speech has changed."

Although Kay Russell has been subject to a degree of media flippancy, she has fared better than the Norwegian woman who was the first recorded victim of the syndrome. Developing a German accent just as the Nazis overran her country, she was ostracised for the duration of the conflict.

Morgellons Syndrome

Morgellons syndrome is the most contentious and wince-inducing condition on our list. Although yet to be officially classified as a disease, those who claim affliction – and there are several thousand in the US – report the presence of subcutaneous parasites which manifest themselves as long, fibre-like worms that move around the body.

However, there is considerable debate among healthcare professionals as to whether Morgellons is a real illness at all. In fact, doctors point out that Morgellons bears all the hallmarks of delusional parasitosis, a secondary phenomenon connected to schizophrenia. With the suggestion being put forward that the "fibrous parasites" can actually be explained much more simply: they are actually fibres from patients' clothes which have become entrenched in open wounds. Sufferers vehemently dispute this contention.

Porphyria

The enzymic disorder porphyria was first recognised in the 4th century BC by the father of modern medicine, Hippocrates. Taking its name from the Greek term for "purple pigment", the disease causes a build-up of toxic porphyrins, which discolour the urine of victims and, in some instances, trigger paranoid delusions. It can also cause pain, cramps and fitting.

The most prominent victim was George III (and porphyria is thought to be responsible for "the madness of King George"). A spartan regime of blistering and leeches was prescribed by the royal physician, Francis Willis. Although the blistering failed to help the King, it helped Willis, who was given a pension of £1,000 a year. The King's case is now thought to have been caused by arsenic poisoning from medication he was taking. "Leeching is not something we'd recommend today," says Dr Joanne Marsden of King's College Hospital. "We prefer dietary adjustment and pain control."

Todd's syndrome

The author Lewis Carroll was said to have been inspired to write Alice in Wonderland after the onset of a bizarre neurological condition. Prey to "bilious headaches", Carroll recorded in his diary his experience of "curious optical effects ... seeing disordered 'fortifications'." Although historical diagnosis is risky, scholars agree that these "disordered fortifications" were the likely product of Todd's syndrome, a condition which distorts perceptions of size, and induces the shrinking feeling Alice experiences. Only formally recognised 60 years ago, it affects just a tiny number of people per year.

Pica

Often manifesting itself in the early stages of pregnancy, Pica leads to an uncontrollable desires to consume certain foods and, in some cases, inedible objects – sponges, lengths of wood, and bricks have all be reported as objects of desire for sufferers, and it's common to be drawn to eating soil, coal or paper.

"There has been the suggestion that the cravings are the result of an iron deficiency, but the connection has yet to be proven," says Sue Jacob of the Royal College of Midwives.

Coulrophobia

Coulrophobia describes a terror of clowns. The condition is suprisingly common. So much so that the music event Bestival had to discourage visitors, many of whom come in fancy dress, from dressing up as clowns, as there were so many potential phobics among the audience. It's not recognised as a medical condition, but commonly affects children.

Other surprising phobias include chorophobia, which is a socially disabling fear of dancing, and gephyrophobia, a fear of bridges. This is a relatively common condition and a potentially dangerous one, as it can induce panic in drivers, which can be quite a hazard in bridge-dense places such as New York. So much so that the authorities are available to guide drivers over the Tappan Zee Bridge, and in San Francisco they are well used to dispatching tow trucks to panic-stricken drivers.

While we may be tempted to smile at the idea of someone plagued with worries about clowns, or who crave inedible objects, as Stephen Nutt from Rare Disease UK points out, the effects can be life-changing. And with one in 17 people developing a rare disease during their lifetime, you or I could be next.