In 1998, gastrointestinal surgeon and researcher Andrew Wakefield, together with a dozen other co- authors, published a paper in esteemed medical journal The Lancet that said the measles, mumps and rubella (MMR) vaccine can cause autism.
The paper, which was eventually exposed as fraudulent, sent vaccination rates going south. Since then, several studies have debunked the link between autism and the MMR vaccine, or vaccines in general.
But many people continue to believe the myth. "People have studied vaccines a lot after that... There's no evidence that they cause autism," said Dr Shyam Prabhakar, associate director of integrative genomics at the Agency for Science, Technology and Research's Genome Institute of Singapore.
"In science, you can never say no. It's hard to prove a negative, but analyses of tens of thousands of people in many different countries have failed to show any link to vaccination," he said.
According to not-for-profit charity Autism Resource Centre (Singapore), there have been no accurate studies conducted in Singapore to establish the exact prevalence rate of autism here. But it is likely that based on figures worldwide, it is in the region of 1 per cent of Singapore's population, which is around 56,000 people. Fewer people now go undiagnosed than in the past, because of greater clarity about the diagnostic criteria, said the charity on its website.
This is coupled with an increase in awareness and knowledge among those who diagnose the disorder, as well as other professionals.
Still, there is more that remains unknown about autism than what is known and no two people with autism are the same. They can present a range of symptoms and different levels of the disorder.
This is also what makes diagnosis tricky, as doctors have to look at a range of behaviours and also examine a child's characteristics in different areas - such as how he interacts with others around him.
There is also no known single cause for autism - also called autism spectrum disorder - but studies have shown that there are certain factors that could increase the chance of a child having it.
For example, when a pregnant woman is down with an infectious disease, her immune response could affect the child's brain development. If the mother is exposed to certain toxins, such as pesticides, the foetus could also be at greater risk.
In addition, there are certain genetic mutations that have been associated with the disorder.
"People have said autism is not one disease but many and the reason is that it is estimated that there are about 100 different DNA mutations and environmental factors that cause it," said Dr Prabhakar.
"That makes life very complicated if you want to design a treatment. Ideally, you want to identify one cause so you can treat it."
Interestingly, autistic symptoms have been observed in genetically engineered mice, noted Dr Prabhakar. These were found during tests used by scientists to determine whether a particular gene causes autism.
For instance, mice that were given the same genetic mutations found in people with autism spent less time paying attention to other mice.
The ultrasonic noises made by mice, which are inaudible to human beings, also changed when such genetic mutations were introduced into the animal.
Carolyn Khew
