I believe many mental illnesses, depression, suicidal behavior and anorexia among them, can be characterized as contagious. And perhaps some other things that aren't usually described as mental illnesses. The border between belief, religion and mental illness seems a bit fuzzy (echoes of Snow Crash). Small isolated communities must be much more susceptible to contagion taking hold, as single events have a great sway over perceptions, and there are few connections with a larger world to normalize things.
Charles Stross said something about the internet that stuck with me: "the accidental invention of telepathy". Just like public water supplies opened up new vectors for diseases such as cholera, I wonder if the internet's ability to more or less put minds directly in touch can facilitate the spread of certain mental illnesses. Consuming unfiltered internet might be looked on in a few years like drinking unsterilized water. In particular, I wonder if certain "echo chamber" social websites that allow easy isolation from mainstream opinion (Tumblr and 4chan, I'm looking at you) might be analogous to these dangerous isolated communities.