Antivaxxers are a major public health threat. These are misguided people -- generally good-hearted but still mistaken -- who think that vaccinations can cause autism in children. This is despite a raft full of evidence showing no connection whatsoever. Basically, antivaxxers resort to anecdotes, misinterpreting data, and an appeal to emotions to make their case.
They also have a lot of support by celebrities, sadly. Jenny McCarthy is a vocal antivaxxer, and she's roped her boyfriend, Jim Carrey, into this as well. They had a big fund raiser for their nonsense recently, and I have to bite my tongue very strenuously to avoid an ad hominem here, but I'll say that Britney Spears and Charlie Sheen attended.
On the other hand, I was very pleased to see the video below at The SkeptTick's site. It shows actor Amanda Peet talking about this topic for Cookie magazine (a parenting mag), and it's very effective. She obviously loves her child, and makes a strong case that we need to be more active in fighting antivaccination nonsense.
I love Ms. Peet; after being attacked by antivaxxers for her comments she held her ground. You need to read what she wrote. It's awesome to behold.
You can search the blog here for more info on the tactics used by the antivaccination crowd. Another resource is the great blog Respectful Insolence, where Orac keeps up with them as well. Have no doubt: the antivaxxers mean well -- many have autistic children themselves, and as a parent I feel for them very much -- but if they are successful then many, many children will die of totally preventable diseases. And we're already seeing a comeback of measles due to drops in vaccination rates. This is no far-flung future; the terrible consequences of antiscience nonsense are happening now.