Nurse Marsha Maitland was sitting by the bed of a very sick 19-month-old little girl in London's Hammersmith Hospital. To watch over a recovering child was not an unpleasant duty, but this child was mysteriously dying.

Her parents had flown her from Qatar, an oil-rich Middle Eastern country, and she had arrived semi-conscious and with her blood pressure falling. The doctors had tried testing the child for several conditions, but nothing came to light. They were baffled.

There was nothing to do but wait now. To pass the time under such unhappy circumstances, Maitland read Agatha Christie's detective novel "The Pale Horse."

From time to time the doctors would huddle by the child's bedside trying to puzzle out the symptoms. The little girl was declining. She became more moribund. Her breathing came with difficulty and she began to lose her hair.

Something about the child's symptoms sounded vaguely familiar to Maitland. "The hair falls out." It was a line from the novel she was reading, a novel in which murders were committed by thallium poisoning.

Could the child have thallium poisoning?

The next time a doctor came in to check on the girl, Maitland suggested her theory, explaining that the child's symptoms matched the ones described in the book.

"We were at the state where almost any suggestions were welcome," recalls Victor Dubowitz, professor of pediatrics at the Royal Postgraduate Medical School and physician-in-charge on the case.

Dubowitz and the medical team turned to Scotland Yard for the address of a medical laboratory that could test for thallium poisoning. The Yard also put them in touch with a thallium expert - an inmate at the Wormwood Scrubs Jail, who kept detailed notes on the effects of the chemical as he poisoned his pet rabbits, his family and some of his coworkers.

When the test came back, the doctors discovered that the child's body contained more than 10 times the permitted maximum of chemical. The doctors also discovered that thallium was commonly used in the child's home area as a domestic poison to kill cockroaches and rodents.

The doctors immediately began treatment and within three weeks the little girl showed "remarkable" improvement Dubowitz says. After four months she was discharged and returned to Qatar with her parents.

"When we last saw her she had made a good deal of progress an was sitting up and taking notice again," says Dubowitz, who wrote up the curious case in an article in the June issue of the British Journal of Hospital Medicine.

"Thallium is so rare, no one in this country would think of testing for it," he says.