That is the ultimate threat. The biological-weapons threat has been exaggerated. A lot of people have been trained all their lives to worry about nuclear war. Now there is less to worry about. They are not particularly attractive weapons, in my view, for terrorists either.
Terrorists prefer weapons that are simple, cheap and reliable and that make a big noise.
There is a real threat of leakage of nuclear-weapons materials, like plutonium or highly enriched uranium. For nuclear-wannabe countries in the world, the biggest roadblock is the access to this material. Only very small amounts are needed. Russia has very large amounts that are under very poor control. The sale or theft of plutonium or highly enriched uranium is certainly a very, very big threat.