There are effective vaccines against smallpox.

I think that if a belligerent were to use the disease as a biological weapon, it would first secretly stockpile large quantities of vaccine. Once the disease breaks out, there would be a window of opportunity lasting a few weeks to a few months during which one could gain an advantage, because the enemy would first have to ramp up vaccine production and vaccination rates. When I think about it, however, I have serious doubts as to whether such preparations in the 21st century could be kept secret enough to actually surprise the enemy (in this case, the West).