Against modern Western nations, viral weapons are particularly effective. Infected people usually won't show symptoms for at least a few days after infection. Since people in Western nations travel a lot, the virus could potentially spread its devastation over a huge area.
Viral weapons are also considerably less expensive. While it would take hundreds of scientists many years and millions of dollars to develop working nuclear technology, a dozen biologists with a few hundred thousand dollars could develop a viral weapon in a matter of weeks. In addition, the weapon could be developed and created using pharmaceutical equipment that is readily available all over the world.
However, Iraq's stock of viral weapons is closely watched, while those of other countries are not. Russia is having what is effectively a yard sale on all sorts of weapons, from conventional weapon systems to rare metals. While they are not known to have sold biological or nuclear weapons, the possibility is definitely there. Their economy has not been stable in the recent years as given example by the ruble crash in August of 1998. If given a choice between selling weapons or starving, most people would forgo their morality for a chance to eat. Scientists in Russia earn a mere $100 per month, yet are relatively well off, suggesting possible corruption.