We're pretty good at disease control, see eg. the recent ebola outbreak. An outbreak of anything dangerous would be quickly contained. There are no organizations with the capacity to surreptitiously produce, distribute and release an infection agent broadly enough that it might kill in the billions before being contained.
That basically leaves the US or Russia (the only countries with the capacity to do so) arbitrarily deciding to nuke most cities in south Asia.
I'd give China more of a chance than Russia, actually. They have more resources. (But perhaps less willingness than Russia: China is doing very well with the status quo.)
I don't think China has the nuclear warhead stockpile to kill 1 billion+ people. Wikipedia says "Current stockpile (usable and not): ~260" which means each would have to kill 3.8 million, which basically means delivering each perfectly into a major city. That's a non-trivial operation to put it mildly.
That said, I don't see any objective (rational or otherwise) of either country that would be advanced by such an attack. And again, we're talking about an attack that will kill 1B+ people, not just any nuclear attack -- that's a different story. Still low probability, but at least there are semi-rational objectives that could motivate it.
Oh, I didn't mean to imply the Chinese had enough stockpiles. I meant they have a big enough economy to rapidly build up their nukes (or invest in bioweapons etc), should they decide to.
