by Alison George
"In this week's issue of New Scientist I edited a profile of a doctor who is the real-life version of TV's House MD. William Gahl recently set up the Undiagnosed Diseases Program to hunt for the answer to mysterious diseases that have defied all other medical experts (read the interview).
This got me thinking about ailments that have perplexed the medical profession. Here is a selection of the most unusual.
This may sound impossible - considering that our bodies are around 60% water - but some rare individuals are allergic to water.
They can still drink the stuff, of course. It's washing that causes the problem. A few minutes in the bath or shower causes their skin to erupt in itchy red weals.
This rare condition (known, medically as aquagenic urticaria), was first described in 1964.
Its cause is still a mystery: it could be due to a toxic response when water touches the skin, or to an extreme sensitivity to ions in the water.
Imagine going for genetic tests along with your children, only to find that you can't possibly be their biological mother - despite the fact that you gave birth to them.
This isn't science fiction. It has actually happened to a few people around the world (see The stranger within).
In the case, it turned out that the mother was a chimera (a mix of two individuals). She was the composite of two non-identical twins that had fused in her mother's womb.
No-one knows how common chimeras are, but with the rise of fertility treatments and genetic testing, more chimeras are likely to be created and discovered.
Foreign accent syndrome
If you wake up talking with a strong Jamaican accent, despite the fact that you've never even heard a Jamaican accent before, then the chances are you're suffering from foreign accent syndrome.
The best known case of this syndrome dates from 1941, when a Norwegian woman was ostracised after she was injured during an air raid and began talking with a strong German accent.
This syndrome was once regarded as a psychological disorder, but it's now thought to be a neurological one, which comes about when a stroke or injury damages the part of the brain associated with speech.
The real mystery is how people talk with an accent they've never been exposed to - though recently scientists have come up with a possible explanation. The "foreign-ness" of a sufferer's accent might simply be due to the listener struggling to interpret the change in the sufferer's speech patterns.
The symptoms: fibres growing out of itchy lesions, accompanied by a biting, crawling sensation, as if the sufferer is being attacked by a parasite.
Thousands of people around the world report these unpleasant symptoms and claim that they're suffering from Morgellons disease - a physical ailment whose cause is, as yet, unknown.
But present these symptoms to most doctors, and they'll tell you you're suffering from a well-known psychological disorder called delusional parasitosis - an unshakeable belief that sensations of crawling on or under the skin are caused by parasites, despite incontrovertible evidence to the contrary.
Who is right? The jury is still out, but some medics are beginning to take a serious look at Morgellons. One suggestion is that the fibres could be the result of an infection by the bacterium Agrobacterium, most commonly found to cause tumours in plants.
The madness of King George
The British King, George III, suffered major bouts of mental derangement, for which he had to be restrained in a straitjacket or tied to a chair.
Scientists thought they knew the cause of these ravings: a genetic defect called porphyria. But in 2005 researchers examining a sample of King George's hair made a surprise discovery: high concentrations of arsenic.
The researchers believe that the medicine given to the King was contaminated with arsenic - making his predisposition to porphyria far worse.
Read about other historical medical mysteries
A 1996 issue of the medical journal The Lancet carried a distressing case study. A 29-year-old man had pricked his finger on a chicken bone 5 years previously, leaving him with an infection which made him smell terrible.
"The most disabling consequence of the infection was a putrid smell emanating from the affected arm, which could be detected across a large room, and when confined to a smaller examination room became almost intolerable," the paper states.
The cause of the infection couldn't be identified, and it didn't respond to antibiotics, so the paper's authors issued a plea: "We ask assistance from colleagues who may have encountered a similar case or for suggestions to relieve this patient's odour even if the organism cannot be eradicated."
So what happened to this poor man? Does he still smell putrid after all these years? I contacted the dermatologist who treated him to find out.
Peter Holt of the University Hospital of Wales reported that the patient's infection cleared up spontaneously, and he no longer smells putrid. But the cause remains a mystery.
With hands and feet resembling branches, Dede, a man from West Java, Indonesia, appears to be half tree, half man.
But what is the cause of this deformation? Thankfully for Dede, this mystery may recently have been solved.
The culprit appears to be a rare immune deficiency, which allows the human papilloma virus - better known as the cause of warts - to rampage out of control."