Some time ago an American public health researcher friend here in London showed me some work her team had done examining the social web an vaccine safety rumours. 

Short version: similar but false messages about vaccine safety popped up from a handful of sources and quick spread virally, leading people to conclude that there must be some truth to information seen in so many places. 

Seems this kind of reaction to / propagation of false or misleading information online is not confined to vaccines. The article below shows similar patterns for political and public security issues and news, too.