The media, of course. Well, mainly. Figures such as Andrew Wakefield and Arthur Krigsman played their parts too and the JABS forumites are well known for being dangerously wrong. There is currently a thread over there relating to the rise in measles and it would be interesting to see the responses. See Struthers, Stone and Fletcher’s rabid responses on the BMJ website here for a rough guide.
Here is the BBC report. Here is a Bad Science post arguing that people really do listen to journalists. The BMJ has a paper on MMR uptake that shows that 9.5% of parents consciously choosing not to immunise did so on the basis of negative media coverage. Other reasons included “being too scared or thinking the vaccine was too dangerous”, “not wanting to their child to receive MMR” and “fears over possible links with autism”. It would not shock me if it turned out that the decisions of many of the parents who gave these reasons were informed by media coverage too. Some bloggers did predict a rise in measles. And now it’s here.