Why do people think vaccines lead to autism?

Unethical. A paper published several years ago linked autism to MMR vaccination. Since then many follow up studies and investigations have disproven it completely. The author of the original study was later found to have altered his results due to having financial gain out of such results. It is a sad case of unethical medical behavior which has caused much confusion.
Fraudulent data. Several studies have disproven the link, which was originally published in 1998 by dr. Andrew wakefield. An investigation by the british medical journal (bmj) concluded that dr.Wakefield faked data and created an elaborate fraud, causing long-lasting damage to public health. In fact, the original study was eventually retracted in 2010, and dr. Wakefield lost his license.
Looking for answers. We want answers. What is it, why did it happen (so we can avoid it in the future) and can it be fixed?Conditions with poor explanation like autism or crib death give rise to speculation & both have been tagged as vaccine related on occasion.As research provides better answers the idea fades.New microarray technology has shown small chromasomal defects in many autism pts.This research is promising.
Misinformation. In the desperate need to find a cause for autism and then, perhaps, a cure, people are sometimes willing to accept false, unprocen and speculative points as being proven facts.Fortunately all of the evidence supports the safety of vaccins and no relation to autism. Immunize.
Faked Research Paper. Dr Andrew Wakefield published an article in the British journal the Lancet connecting the MMR vaccine to autism. The news agency ran with the story and caused great fear/concern. Other scientists could not duplicate the results and later it was found Dr. Wakefield falsified his research for monetary gain. He was stripped of his license in England and the article was retracted. http://www.parents.com/blogs/to-the-max/2012/03/27/autism/autism-vaccines-and-the-fake-research-that-wont-die/