Liz Birt sits with her autistic son Matthew, 9, on the stairs of their home in Wilmette, Ill. Birt is very active with parents groups that believe vaccines cause autism. Her son developed autism symptoms gradually after receiving two childhood vaccinations on the same day after he was 15 months old. (AP /Aynsley Floyd)

CHICAGO (AP) - Mention autism to parents, doctors and scientists these days, and among an earful of different theories will emerge a common nod of agreement: the perplexing condition is not nearly as rare as once was thought.

As recently as a decade ago it was estimated that only about four out of every 10,000 children in the United States were affected. Research now suggests the rate may be at least 10 times higher.

The numbers have fuelled debates over whether there's been a true surge of cases and whether environment or genetics could be the cause. Some parents and research advocates blame vaccines despite recent evidence to the contrary.

But many mainstream scientists point to two less worrisome explanations: the definition for autism has changed and schools now offer more educational services to autistic children.

In 1991, the U.S. Department of Education made autism a new, separate category for special education services offered at public schools. Those services tend to be broader and more intensive than for other disorders, including mental retardation. There's evidence that the 1991 change prompted what some call "diagnostic substitution," said Dr. Fred Volkmar, a Yale University autism researcher.

"Autism is kind of a fashionable diagnosis," Volkmar said. "Everybody's interested in getting better services."

The change in school services and the definition, along with research showing that early intervention could help, raised awareness of the condition as something other than "the kid who sits in a corner watching the record player go around and around. Everybody said that's what autistic is and anything else is not," said Chicago pediatrician Dr. Joel Schwab.

Now, autism is increasingly recognized as "being more than just the classic picture," said Schwab.

As a result of the rise in diagnosed cases, schools are struggling to provide enough services to affected children, funding for research into causes has grown, and lawsuits blaming vaccines are proliferating.

"There's just so many kids who have been affected, it's hard to find somebody who doesn't know somebody who has a kid with autism," said Liz Birt of Wilmette, Ill., whose nine-year-old son, Matthew, is autistic.

Within seven blocks of their suburban Chicago home, five other children also are afflicted. "It's just rampant," Birt said.

Autism even ended up in a debate over a last-minute provision attached to Homeland Security legislation enacted by the U.S. Congress last fall. The provision, aimed at protecting drug makers from lawsuits over vaccine-related injuries, prompted vocal protests in Washington in January by parents who think childhood vaccines cause autism.

Much has been learned about autism since it was first described in the 1940s by psychiatrist Leo Kanner as a distinct developmental disorder, but many key questions remain, including what causes it.

Some think environmental factors might trigger the disease in genetically susceptible people. Potentially plausible but unproven triggers range from illness during pregnancy to soil toxins, electromagnetic waves and even vaccines, though strong evidence so far suggests the shots are safe.

Kanner described what is now known as classic autism - children with severe impairments in language and communication, who may appear deaf, sometimes don't speak, show little eye contact and appear more interested in interacting with objects than with humans. Repetitive behaviours such as rhythmic finger tapping or ball-rolling are common.

Sometimes symptoms show up in children who previously appeared to be developing normally; some call this regressive autism.

Autism is not curable but can be helped with behavioural treatment and sometimes medication. Recent research has shown that intensive behavioural training starting as early as infancy can substantially improve symptoms in some autistic children.

"People have a much better idea about the diversity of autism. There's such a range of both severity . . . language handicap and mental retardation," said prominent autism researcher Catherine Lord.

"That has changed perspectives on autism both in terms of figuring out who needs services and also the prognosis for people in the mild range," said Lord, director of the University of Michigan's Autism and Communications Disorders Centre.

Not all children with autism are mentally retarded but most need special services.

Kathy Gould, project director for an Illinois program that trains teachers and parents how to work with autistic children, said demand has increased significantly in the past five years.

"Parent workshops have gone from three a year to 15 a year. Parents are crying out for additional help," she said.

Liz Birt is among them.

Her son, Matthew, developed normally until he was 15 months old, when he could count to 10 and say about 30 words. He developed autism symptoms gradually after receiving two childhood vaccinations on the same day, Birt said. He stopped talking, acted as if he was deaf, spun in circles, stared at lights and shunned his family.

At nine, Matthew Birt is still profoundly affected and his mother worries that as he grows into adulthood, no services will be available.

"Somebody's got to pay attention to this," Birt said. "We're talking about hundreds of thousands of children who are going to be a big drain on the economy."

Vaccine foes like Birt point to a 1998 British study linking autism with the measles-mumps-rubella vaccine. There's been a subsequent backlash against vaccines in England - and recent measles outbreaks.

The Institute of Medicine reviewed the issue and in 2001 said there was no proof that autism is caused by the MMR vaccine or by the mercury-containing preservative thimerosal that was present in some vaccines.

Vaccine foes note the IOM report said a link between thimerosal and an increased risk of neurodevelopmental disorders is "medically plausible."

Dr. Neal Halsey, an influential vaccine proponent from Johns Hopkins University, agrees that thimerosal could theoretically be linked with subtle developmental problems including delayed speech, "but the available data show no evidence of an association with autism."

While Halsey worked to have vaccine makers discontinue thimerosal use after evidence in 1999 suggested that the combined amount of thimerosal in vaccines could expose children to mercury exceeding recommended maximum levels, he says he never believed that any vaccine caused autism.

Ironically, the opposite may be true, he said, since women who develop German measles (rubella) early in pregnancy face an increased risk of having an autistic child.

Volkmar, at Yale, and colleagues have found that while healthy babies learn social interaction by focusing their gaze on people's eyes, autistic children focus more on mouths.

This could help parents and doctors identify affected children earlier and get them early intervention, Volkmar said.

Other scientists are searching for genes and other "biomarkers" that might make autism as easy to diagnose as a simple blood test. For now, doctors still rely on behaviour to diagnose autism.