Story highlights David M. Perry: The murders of a teacher and student are already fading from the headlines

But we should not forget what happened to Karen Smith and Jonathan Martinez

David M. Perry is professor of history at Dominican University in Illinois. He writes regularly at his blog: How Did We Get Into This Mess? Follow him on Twitter. The opinions expressed in this commentary are his.

(CNN) Only four victims.

In San Bernardino, California, this week , an armed man burst into a classroom with a gun and began firing. As the news of another mass shooting in a school rolled over the country, visions of Sandy Hook Elementary School, where so many died, flooded into my mind. When I heard only four people had been shot, I relaxed a little.

David M. Perry

There's something wrong about being relieved by a small number of people being killed. It's a sign that the constant barrage of murder stories have begun to harden one's empathy. It's a defense mechanism and it worked for me at first, helping me get through my day. Later my horror returned, accompanied by guilt at my previous relief, as we learned the killing had taken place in a special education classroom.

My son is a 10-year-old boy with Down syndrome. He spends part of every school day in a special education classroom. He panics at loud sounds, placing his hands over his ears, crying, or just saying "no." It was too easy to imagine him there in San Bernardino. The new detail made the violence suddenly visceral.

The murder of teacher Karen Smith and 8-year-old Jonathan Martinez, one of her students, shines a spotlight on at least four of the major correlating factors that increase the risk that a person will suffer violence: disability, domestic abuse, race, and guns.