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Sergei Popov.

Genetically engineered Smallpox, Soviet Union.

(Really scary! First pass, 70-80% fatality rate, second pass bumps it up to 90+ No vaccines, and damn few treatments once infected)

Wired did an article on BIOWp

The most fightening thing about Bio Weapons is the ease of creating them. A few thousand bucks worth of equipment in a space the size of your average two car garage. A working knowledge of biology, and the right bacterial or viral cultures. The hard part is obtaining the really nasty seed stock. Anthrax is always in the news, but it ain't the worst. What if someone created an easily transmitted varient of Ebolla? I'll leave you to your own nightmares about the possibilities.