A great post from Making Light about why we immunize.
UPDATED 2009-02-24 to add: Phil Plait at Bad Astronomy links to what he calls "the be-all and end-all of posts about the antivaxxers". It's a great post, and would be so even if all it did was summarize all the bad science and bad journalism that went into the making of Andrew Wakefield into the famous fraud he is.