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Mostly quiet since his last birther spasm, Donald Trump today went on Twitter to peddle a junk science theory that claims vaccinations cause autism in young children.

"Massive combined inoculations to small children is the cause for big increase in autism," Trump wrote. "Spread shots over long period and watch positive result."

The Romney campaign, for whom Trump has raised millions this campaign season, would not comment on his latest offering.

Doctors and medical research findings, however, are less circumspect.

Dr. Richard Besser, ABC News' chief health and medical editor, called Trump's remarks "shameful."

"The autism-vaccine link has been disproven. Spreading shots out over a long period of time will not reduce the number of children who develop autism but it will leave more children vulnerable to infectious diseases for a longer period of time than necessary," he said. "That can kill children."

While it's true that autism diagnosis rates have risen over the years, there is an ongoing debate over whether or not the numbers can be taken at face value. Many doctors believe a broadening of diagnostic criteria has led to more confirmed cases.

What is unquestioned, though, and confirmed by serious medical studies, is that there is no known connection between the condition and having received childhood vaccinations.

"As we know from political campaigns, stating a claim repeatedly can lead to a public belief in the concept since these conclusions are not always based on rational thought processes but also on emotional thinking and preconceived notions," Dr. Max Wiznitzer, associate professor of pediatric neurology at Rainbow Babies and Children's Hospital in Cleveland, Ohio, told ABC News in March.

The scientific paper that once served as a the driving force behind the theory has long since been discredited and rejected by its original publisher, The Lancet, which wrote in 2010 that "it has become clear that several elements of the 1998 paper by [Andrew] Wakefield et al are incorrect.

"In particular, the claims in the original paper that children were 'consecutively referred' and that investigations were 'approved' by the local ethics committee have been proven to be false."

The British Medical Journal published an editorial in January 2011, calling the Wakefield report "fraudulent," adding that "clear evidence of falsification of data should now close the door on this damaging vaccine scare."

Closed, that is, until Trump opens it again.

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