Katie Couric's Notebook: Nuclear Weapons

Here's a frightening prediction by Harvard professor Graham Allison.

He says within the next five years, it's more likely than not that terrorists will launch a successful nuclear or biological attack somewhere in the world.

That's why more than 40 world leaders have gathered in Washington this week to discuss how to prevent al Qaeda and other terror groups from getting nuclear weapons.

The summit comes on the heels of last week's landmark agreement in which the U.S. and Russia agreed to cut a third of their nuclear warheads.

Though each nation will still have enough to destroy the world many times over, both sides did agree to keep talking about additional cuts.

But here's another sobering thought:

Two decades after the end of the cold war, Russia's massive nuclear arsenal may pose less of a threat to our security than one loose nuke that falls into the wrong hands.

That's a page from my notebook.

I'm Katie Couric, CBS News.