can do to strengthen deterrence. >> yang: joseph cirincione, matthew kroenig, thanks for joining us.you. >> woodruff: next, a long looked-for breakthrough in the battle against the deadly ebola virus. one experimental vaccine has been shown to be 100% effective on humans. hari sreenivasan has the story. >> reporter: the results of a two-year trial led by the world health organization were published in the journal "lancet" yesterday. they showed that of nearly 6,000 people vaccinated in guinea last year, none had contracted ebola after a ten day period. while in the group not vaccinated, 23 cases developed. the vaccine still needs regulatory approval, but is considered so effective that some 300,000 doses have already been stockpiled. the 2014 ebola epidemic in west africa killed 11,000 people. for more on this promising news, we turn once again to dr. anthony fauci. he's director of the national institute for allergy and infectious diseases, and has long been involved in vaccine development. dr. fauci, everyone's pretty excited about the 100% efficacy rate. let's look under the hood

joseph cirincione is the president of the global security foundation ploughsha res fund.ies could have done more to prevent this attack. terrorism of this kind is extremely difficult to prevent. was this individual... did he commit the act because we didn't deport him soon enough, or was he disturbed individual who committed the act because he was about to be deported? we don't know yet. but we do best. ever since the terrorist attacks of 9/11, national security experts including myself have been focused on the next big, catastrophic terrorist attack, another well—planned, highly co—ordinated attack, perhaps involving nuclear weapons or dirty bombs. well, it turns out that is not the kind of terrorist attack we actually have to worry about. it is now this spread of low—tech terrorist attacks, dispersed, autonomous, inspired attacks. it turns out that knives and semiautomatic weapons and home—made bombs, and our trucks, are perfectly adequate for causing the kind of terror and societal disruption that the terrorist seek. something like this, an individual, maybe acting on