A test case in an American court could in one way or another settle the debate on the possible link between autism and childhood vaccines.
In an unprecedented legal challenge in a special court in Washington, the Court of Federal Claims, the case of a girl from Arizona, is the first to be heard of more than 4,800 cases.
The cases have been filed by parents who believe their children's autism was caused by a mercury preservative in the vaccines.
Michelle Cedillo, a 12-year-old, suffers from a plethora of health problems, including severe autism, inflammatory bowel disease, glaucoma and epilepsy which her parents believe are the result of the MMR vaccine she was given at 15 months.
The parents want compensation from the National Vaccine Injury Compensation Program, a no-fault system with a $2.5 billion fund made up from a 75-cent-per-dose tax on vaccines.
This first test case will question whether the combination measles, mumps and rubella (MMR) vaccine, plus a mercury-containing preservative called thimerosal, caused Michelle's autism.
As vaccine experts point out parents often link vaccines with their children's symptoms because vaccinations can be upsetting, and children are vaccinated at an age when autism and related disorders often first appear.
A solid body of scientific evidence challenges the parents cases and medical experts and the Government have repeatedly assured parents that the MMR vaccine is safe.
According to the World Health Organisation a 95 per cent vaccination coverage of children is necessary to achieve "herd" immunity, where sufficient numbers are protected to prevent outbreaks and epidemics of disease.
But controversial research by Dr. Andrew Wakefield published in The Lancet in 1998, linking the MMR vaccine with autism and bowel diseases has fuelled the cause and public confidence appears to have been affected.
A report earlier this year found, in some parts of the UK, as few as one in nine children were being given the triple vaccination.
Since 1998 two Institute of Medicine reports in the US, in 2001 and 2004, determined there was no link between vaccines and autism.
Experts believe the debate has been settled and say a scientific question is best answered in a scientific venue and not in a court.
The experts say that although thimerosal was removed from all childhood vaccines in the United States, except flu vaccines, by 2002, rates of autism continued to climb.
The U.S. Centers for Disease Control and Prevention (CDC) estimates that around one in every 150 children has autism or a related disorder such as Asperger's syndrome and about 560,000 people under 21 in the U.S. have autism.
Autism is characterized by impaired social interaction and those affected often have trouble communicating, and can also exhibit unusual or severely limited activities and interests.
Classic symptoms of mercury poisoning include anxiety, fatigue and abnormal irritation, as well as cognitive and motor dysfunction.