Looking back, it was the summer of '17 when the wheels really came off.

Some people say things started the year before, during the presidential campaign, when the man-child's popularity grew with every insult, every crude remark, every sullen poke in the eye of a calm and rational world. And sure, there were some incidents—at his rallies, mostly. Punches thrown. Bigoted taunts shouted. The man-child himself wishing aloud he could bash somebody in the face. Things like that.

Even so, the country pretty much held it together that year. The guy who sucker-punched another guy at a rally for the man-child apologized when they met in court, and the second guy even hugged him. The man-child won the election, and a lot of people cried. But nobody went postal over it, not even after the inauguration.

Then the warm seasons came and everything seemed to blow up.

It started on the campuses. Sheltered children of privilege started acting like cultural revolutionaries, rioting against ideas they didn't want to hear and assaulting people who expressed them. They'd never heard of the "struggle sessions" in China under Chairman Mao, but that's exactly what they were up to. It was a kind of madness.

And it was contagious. People showed up at town-hall meetings with congressmen just so they could boo and heckle and scream "F—you!" From Berkeley to Boston, opposing political tribes got into repeated, violent confrontations. At one of them, a professor attacked people with a bike lock. A senator's son was charged with three misdemeanors related to mayhem at another political rally in Wisconsin.

In Montana, a congressional candidate body-slammed a reporter for having the temerity to ask him a question. This was on the eve of a special election, and in a more sensible time that would have been the end of the candidate's career. But America woke up the next morning and found out he had won. A lot of people actually kind of liked what he did.

A few days later in Portland, a man on a train—a man named Christian, of all things—started screaming anti-Muslim insults at two women. Three other passengers intervened, so Christian stabbed them. Two of them, Rick Best and Taliesin Myrddin Namkai-Meche, died. As he was being carried away on a stretcher, Namkai-Meche said, "Tell everyone on this train I love them."

At that point, you might think the nation's citizens would have taken a collective step back. Drawn a few deep breaths. Looked around with bewilderment in their eyes, as if they had just woken up from a trance. Stopped and wondered just what the hell, exactly, was going on.

Nope. Scarcely before the blood in Portland had dried, crowds of protesters poured into the halls of the Texas state legislature, lawmakers got into scuffles on the Texas House floor, and one representative—who later said he had been threatened—told another, "I'll put a bullet in your head."

Maybe it was inevitable. When government plays only a small part in the ordinary person's life, then what happens in the world of politics is of only modest interest or concern. But "as government seeps into every facet of life," one contemporaneous observer wrote that summer, political debates become "extremely high-stakes… If we want to bring political life and discourse in the United States back from the boiling point, we have to make politics less important. When politics and policy matter less, the silly opinions of other Americans… will be far less likely to set us against each other in the streets."

That was part of it, certainly. But there was more to it than that, because it wasn't just America. In North Korea, the reigning madman bent every effort to the development of nuclear missiles. In Venezuela, the regnant goon sent out his security forces to beat up the starving citizens in the streets. A savage religious zealotry that had begun in the Middle East and spread to Europe jumped another barrier and reached the Philippines. You could watch it progress in real time, like aggressive gangrene sending tendrils up an infected leg.

Shortly after the turn of the century a movie called "28 Days Later" had come out. It was a zombie flick, the kind with zombies that move so fast they can run you down almost before you realize they're close by. In the movie, the zombies have been infected by a man-made virus. It's extremely contagious, and it causes uncontrollable rage.

It seems like a pretty good metaphor for ISIS, that movie. And for whatever sickness infected America that summer of '17—the summer when people no longer agreed to disagree. No longer decided to live and let live. The summer when they decided the only things worth saying anymore ought to be said with a middle finger or a clenched fist.

Nobody really wanted it that way. But nobody knew how to stop it, either.

This column originally appeared at the Richmond Times-Dispatch.