The following is adapted from an e-mail I recently sent. The response I got was... "interesting".
I am a scientist by training. When asked scientific questions, I tend to answer them as a scientist. Scientific questions, like the question of whether vaccines cause autism, are not about belief. They are about evidence and facts -- and the fact is that the courses of action suggested by the anti-vaccination movement kill people. Worse, the advice and propoganda in question kill people, and not only the people who take it. This is why I take the time to study something before I comment on it or talk about it. I'm aware that bad advice and bad information kills.
I am not joking about this. I consider this sort of thing to be a (literally) deadly serious matter. I also don't make unqualified or categorical statements in this sort of matter without a lot of very strong evidence.
England has recently seen a resurgance of measels, resuling in considerable suffering and death. Australia has seen a tragic pertussis outbreak. A very large number of illnesses and deaths here in the US can be directly attributed to vaccination refusal. This is without breaking herd immunity -- and the thought of that happening (what happened in a small segment of Australia), frankly, scares me.
In addition to this, this sort of thing creates and exacerbates the problem of what are known as nocebo and harmful observer-expectency effects. To give you an illustration of how powerful nocebo effects can be, chemotherapy trials have been known to cause the control group (who are not recieving real drugs) to lose hair, start vomitting, and generally suffer all of the negative side-effects of chemotherapy... despite having only been given a saline drip.
This happens purely because said controls expect it to. It's not an effect of the drug, which they haven't been given.
Now, if this can happen with a saline drip (which is about as harmless as it gets), just what makes people think vaccines are immune from this? Things get worse when harmful observer-expectancy effects are factored in. Expectancy effects have killed people.
Of course, all of the above consequences could be forgiven if there were a legitimate danger the anti-vaccination groups were warning against. Leaving aside just how insulting the "don't vaccinate your kids because vaccines cause autism" rhetoric is (and how, exactly, do you think it makes autistic individuals feel when they see parents going on national television and advising other parents to risk both their children's lives and the lives of other people's children rather than risk having a child like them?), there is also the matter of the truth of these claims, which are pretty much universally bunk.
To provide an illustration of just how bad it is, the e-mail which provoked the original version of the above linked me to this website. I didn't have time (or the stomach) to go through and debunk everything said there, but I did provide an example to give her some idea of just how bad its information is.
The site's first citation to "prove" that flu shots are dangerous (and thereby presumably the one they regard as "best") is a paper by Mark and David Geier, which was published in 2003 in the Journal of American Physicians and Surgeons.
The authors of that study are infamous quacks who are guilty of academic fraud and who make their living by chemically castrating children. The "journal" it was published in is an infamous hotbed of quackery which, among other things, has published politically-motivated articles claiming that abortion causes breast cancer and that shaking your baby is safe (and that shaken-baby syndrome doesn't exist), racist articles with utterly absurd claims about illegal immigrants (e.g. this one), and articles condemning both the practice of science-based medicine (e.g. this one) and peer-review (e.g. this one).
And this is without going into the methodological quality of the article (poor) or any of the scientific tactics I'd use to demonstrate that the article was utterly, totally meaningless.
That is just one example. Think about it.