Autism activists planned a rally in front of the U.S. Capitol on Wednesday, to press their contention that the government has covered up evidence linking autism to a mercury-based product once used in vaccines, according to Reuters.

They will also demand more research into autism, which they say is a growing problem.

"There has been a renewed interest in the issue of vaccine safety, particularly on the issue of whether the preservative thimerosal ... may be linked to the occurrence of autism in children," Dr. Julie Gerberding, director of the U.S. Centers for Disease Control and Prevention, told the news conference.

Autism is a mysterious illness that is often diagnosed between the ages of about 18 months and four years; just the time that toddlers and young children are getting their vaccines.