Even though autism affects about 0.6 percent of the U.S. population, its cause or causes remain unknown.

Some scientists have tried to link an apparent rise in the incidence of autism to an increasing number of childhood vaccines that used to contain thimerosal. Thimerosal is an organic mercury compound (ethylmercury) that for a time was used as a preservative in vaccines against diphtheria-tetanus-pertussis, Haemophilus influenzae, and hepatitis B.

In 1999, the U.S. Food and Drug Administration released a statement indicating that some infants who received multiple injections of vaccines containing thimerosal might have gotten enough ethylmercury to exceed recommended guidelines. Shortly thereafter, the American Academy of Pediatrics and the U.S. Public Health Service jointly recommended reducing or eliminating thimerosal in all vaccines.

Since July 2000, U.S. vaccines for hepatitis B, Haemophilus influenzae type b, and diphtheria-tetanus-pertussis have contained no thimerosal, while other vaccines — such as those used against measles-mumps-rubella and chicken pox — never contained thimerosal.

And yet, even though vaccines containing thimerosal are no longer used, studies have revealed that the apparent incidence of autism continues to rise. This increase may possibly be due to better recognition and diagnosis on the part of health professionals.

Consider these facts:

• The symptoms of organic mercury poisoning do not resemble those of autism.
• No excess of mercury has ever been found in the hair, blood, or urine of patients with autism.
• A review of the evidence by the highly regarded Institute of Medicine concluded that thimerosal-containing vaccines have no causal relationship to autism.
• And a new study, published in the Archives of General Psychiatry, has found that the rate of autism continued to rise in California after 2001, the year when nearly all thimerosal was removed from vaccines.

All of these scientific studies and conclusions have failed to convince some people that thimerosal is not the cause of autism. During the past decade, these anti-immunization advocates and parental groups have used blogs, Web sites, Congressional lobbying, and lawsuits against the government to promote the theory that autism results from thimerosal-containing vaccines.

Even some scientists have joined the ranks of those who believe in a link between thimerosal and autism. On the other hand, some of those scientists who have publically denied a causal role for thimerosal have received hate emails and even death threats.

This situation is yet another example of how difficult it is to use scientific evidence to persuade people to abandon their own unproven health beliefs, especially in such a highly charged and emotional issue as autism in one's child.

One concern is that some parents may avoid vaccinating their children altogether, placing them at greater risk from the diseases prevented by the vaccines (not to mention the consequence of their child being barred from public schools).

Quite recently, several scientific reports have demonstrated that a small fraction of the people with autism have abnormalities in genes found on chromosomes 16 and 7, suggesting a genetic cause. Yet, the cause of most cases of autism remains unexplained.