Edna Foa in TIME's 2010 Top 100

Human beings are hardwired to fear things — the lion in the grass, the assailant in the alley — and if one of those fears gets realized, we may never settle down again. The pain associated with that condition is known as posttraumatic stress disorder (PTSD), a mix of depression, anxiety, anger and isolation. No one is doing more to end that suffering than psychologist Edna Foa, 72, of the University of Pennsylvania.

The severity of PTSD is matched by the horror of the things that set it off — rape, childhood sexual abuse, natural disasters. And then there's war. Half a million veterans from the Vietnam era alone may suffer from PTSD — and up to 300,000 from the wars in Afghanistan and Iraq. Too many cope with the disorder by working to exhaustion, drinking to unconsciousness and never, ever talking about it.

Foa, who studied first at Bar-Ilan University in Israel and later in the U.S., always found that unsatisfactory. Thirty years ago, she began studying post-rape trauma, cobbling together therapeutic bits from other anxiety disorders. But it wasn't until 2000 that it all began to come together. That year, she and her husband visited Israel on sabbatical. Just five days after they arrived, the second intifadeh began. It was a crucible for trauma, and that got Foa focused specifically on combat-related PTSD.

The therapy she developed is known as prolonged exposure, or PE, and involves identifying thoughts and situations that trigger the most fear and then gently exposing sufferers to them. Patients first summon up memories of the trauma. Next comes physical exposure to places or circumstances that call it up more vividly. Facing the memories strips them of power. The approach not only works; it works fast — usually within 12 sessions.

The U.S. military — which isn't easily wowed — is embracing PE. The Department of Veterans Affairs is putting the protocol into wide use and implementing programs to teach it across the various services. "The national rollouts are historic," says psychologist Joan Cook, of Yale University, who is studying PTSD in veterans of the Korean War and World War II. "They are unparalleled in the mental-health field."

To Foa, spreading the word is what matters most now. "If you develop a wonderful protocol, it's useless if nobody uses it," she says. There's little danger of that — as hundreds of thousands of service members may one day be able to attest.