Post traumatic stress disorder affects individuals after they see or experience a traumatic event. But experts say that the disorder can also affect communities collectively, at times of tragedies like Newton, Conneticut.

James Hawdon, a professor of sociology at Virginia Tech, says 10-15% of people, in a community where these mass tragedies happen, will display systems of traumatic stress.

Typically, these symptoms include reliving the event; avoidance or lack of emotion; difficulty concentrating; sensitivity and sleep trouble. Physical effects include increased heart rate, fainting and headaches.

But there is a wider effect. "The entire community often defines itself as being the target, as being the victim."

Hawdon says communities go through several stages of recovery – kind of like the “5 stages of grief”.

During stage 1, people express their grief and sympathy. The tragedy dominates conversation.

Then broader issues are addressed, like how to prevent these tragedies from happening again. Froma Walsh, the co-director of the Chicago Centre for Family Health, says this is an important part of recovery.

The final stage is when people stop talking about what happened. But Hawdon says this can cause a struggle if others want to keep debate open. He also says the level of controversy depends on how tight-knit the community was beforehand.

He suggests that the more people interact with each other the better, and that even Facebook and Twitter can help – but only if they are balanced with face-to-face interactions.