Yes its a bad thing to watch Fox Friends and Family. :) But they were just chatting about a possible new threat from Al-Qaeda. A plan to strike a blow in England that would rival Hiroshima and Nagasaki.
First off by using those names, we must assume a nuclear component. If people were talking about an anthrax threat, then we would hear comments about the Soviet anthrax accident that killed dozens of people.
So we start with nuclear material. Where does that leave us? A bomb is possible but still something a wee bit tricky to smuggle in. Iran was mentioned as a possible supporter. Now if Iran has produced a bomb and only one bomb, would they give their sole guarrantor of security from nuclear retaliation to Al-Qaeda? The mullahs are mad but not stupid. Or they could have multiple bombs then all bets are off and so is any last shreds of relevance of the United Nations. Assuming the UN monolith is not vaporized along with lower Manhatten by such a bomb.
So right now the bomb option is on the margins of the table. What is left? Dust is what is left. Radioactive dust composed of uranium or plutonium. Now most envision this dust being packaged as part of a dirty bomb, a conventional bomb that has this dust wrapped around it. When the bomb detonates, the dust gets spread out and breathed in by the survivors. Causing death later on. There is another method of delivery that might sneak past people. Have your operatives get hired by the subway/tube maintenace crews and as they inspect the tracks, sprinkle the dust out. Then let the train's own speed in an enclosed space push the dust before them to contaminate the commuters before anyone becomes aware that something catastrophic has happened.
"The ultimate in disposing one's troops is to be without ascertainable shape. Then the most penetrating spies cannot pry in nor can the wise lay plans against you." - Sun Tzu, The Art of War.