An infant, known only as "Baby Z," in Melbourne, Australia given no chance of survival has amazed doctors after being saved with one of the biggest long shots in medical history.

Baby Z's brain started virtually dissolving soon after she was born 18 months ago because she had too much toxic sulphite in her system.

But her parents and doctors refused to give in to the one-in-a-million genetic condition and stumbled on a highly experimental drug from Germany that had only been tested on mice.

The Herald Sun reported that treatment began a month after she was born and within days Baby Z literally "woke up" from the nightmare condition.

"It was really like awakening — it was just bang, and she was switched on," pioneering neonatologist Dr. Alex Veldman said.

Baby Z's overjoyed mother said she had grown into a happy and determined little girl.

"She is absolutely delightful and as stubborn as anything — I don't know where she gets that from," she said. "She has just started saying a few words and is constantly moving around. Every day just gets better and better. We look at her every day and just think, 'Wow'."

Baby Z is believed to be the first person to be cured of molybdenum cofactor deficiency — a fatal condition that poisons the brain, resulting in seizures and death in early childhood.

Baby Z was treated at Monash Children's at Southern Health in Australia.

The child and her parents cannot be named for legal reasons and to protect their privacy.

Soon after she was born in 2008, Baby Z's toxic sulphite levels were almost 30 times higher than normal and were dissolving her brain.

After three weeks looking for answers, biochemist Dr. Rob Gianello found a research paper by German plant biologist Gunther Schwarz describing how he had developed an experimental drug that was able to save mice with the disease in 2004.

The drug had only been used in animals and nobody knew what it would do in a human.

But Monash's Veldman contacted Schwarz in Cologne and appealed to the hospital's ethics committee to use the drug on Baby Z.

The long shot was backed because the only other option was a painful death.

A Family Court's approval was required to allow the unique treatment to go ahead. Within an hour of the court's approval, Baby Z was given the drug.

Just hours of receiving her first daily dose of cPMP (cyclic pyranopterin monophosphate), tests showed Baby Z's sulphite levels immediately dropped from near 300 to below 100. Within three days they fell to the normal level of about 10.

Baby Z's neurological development is delayed due to some brain damage in the weeks it took to find the cure, but she is now improving.

Click here for more on this story and to see a picture of Baby Z at news.com.au.