Tim Ferriss, on the Joe Rogan podcast [1], predicts a pandemic in the near future (around 28:45). Basically, he knows a guy who runs a biotech company who claims that they, if they wanted to, could engineer a virus within six months that could wipe out humanity.

I wouldn't say that it's the sole threat, however. Yudkowsky seems to take the threat of runaway AI quite seriously. These are crucial times.

[1] http://blog.joerogan.net/archives/3531