Actually, an engineered virus (like the "White Plague" by Frank Herbert, who wrote more than just Dune) or the "Satan Virus" from that movie "Angels and Demons" or "Super Ebola" from the Tom Clancy book seems most likely. Way easier to do and much faster.

In my models I keep getting 100% likelihood that at least 1 and as many as 4 attempts at this will be tried over the next 10-15 years. It's just so cheap compared to all the other options. All you really need are smart motivated people.

In Herbert's book, one guy does it. Herbert lays out really clearly a "how to" path for cooking up and spreading a plague. "Spoiler Alert" you infect $100 bills and mail them to people around the world. No one ever throws $100 in the trash.

This is particularly something smaller countries could do. It's a form of asymmetric warfare that could also be used as a bargaining chip like NK's nukes. Nukes are expensive and take decades to really pull off. A "doomsday" virus could be done in 3-5 years in an undetectable facility.

That's not even considering fanatics and billionaires' who would try and "save the planet" by doing this. When you look at the pressures and the growing chatter about how "emissions" aren't the problem, "overshoot" (ie too many people) is the problem.

I think a major viral attack on the global population is a certainty in the next ten years. Keep an eye out on outbreaks.