If violence spreads like an epidemic, then techniques developed to control TB, Cholera and Aids can also be applied to control and reduce the spread of violent behaviour.
It seems to work rather well.
More controversially, one can easily imagine this technique being used to control and manage the spread of other undesirable behaviours or ideologies: Religious extremism, for example.
To do this efficiently, however, you would need to understand the network of influence that connects individuals: You would need to build a social graph for the whole at-risk population, so you can identify "carriers" and isolate them, as well as identify the group of "at risk" individuals who have been exposed to the ideology; so preventative workers can try to contain the outbreak.
Of course, this would work for any "infectious" ideology; not just religious extremism, but also any arbitrary political ideology or system of belief that follows the "epidemic" model of transmission.
The implications for political campaign work are obvious. You would need to target your efforts, not on people already predisposed to your cause, but on political activists working for the opponent; and on the individuals in their circle of influence.
In other words, you would need to concentrate on controlling the spread of the opposing ideology, not only on promoting the spread of your own.
Naturally, this all seems a bit sinister and manipulative, but if it works and is effective, then I can quite easily see it being adopted, if it is not already standard practice.
I feel kinda sorry for Clapper - I guess he is just trying to do his job and duty, as best he understands them. Perhaps there should be a court case:- The precedent would be a good one to set. However, I don't think that he personally deserves the hatred that he gets in these forums. Criticism, yes; pointed criticism even, fair enough ... but lets' not cross the line into abuse. Besides which, we will need the agencies technical assistance to unravel all of the persistent and ubiquitous surveillance capabilities that we have created over the past decade, and to protect us all from the efforts of overseas agencies and organisations; entities that do not offer up even a pretence of oversight and restraint.