Kaitlyn Greeley burst into tears when a car backfired the other day. She’s afraid to take her usual train to her job at a Boston hospital, walking or taking cabs instead. She can’t sleep.

“I know this is how people live every day in other countries. But I’m not used to it here,” said Greeley, 27, a technician at Tufts Medical Center who was on duty Monday when part of the hospital was briefly evacuated even as victims of the blast were being treated in the emergency room.

Anger, crying jags and nightmares are all normal reactions for both survivors of the Boston Marathon bombings and witnesses to the mayhem. While the injured and those closest to the blasts are most prone to psychological aftershocks, even people with no physical injuries and those like Greeley might feel the emotional impact for weeks afterward as they struggle to regain a sense of security. What’s not clear is who will suffer lingering anxiety, depression or even post-traumatic stress disorder.

But specialists say that how resilient people are helps determine how quickly they bounce back. The resilient tend to be people who share their emotions before becoming overwhelmed, who know how to cope with stress, and who have the ability to look for a silver lining – such as focusing on bystanders who helped the wounded.

Focusing on the horror, “that’s harder on our body and our mind,” said Dr. Catherine Mogil, co-director of the family trauma service at the University of California, Los Angeles. “People who tend to be able to make positive meaning out of tough situations are going to fare better.”

Among the typical reactions that psychologists say anyone who witnessed the bombings or their aftermath might experience include difficulty sleeping or eating; sweats or stomachaches; anxiety or fear – especially in crowded situations that remind people of the bombing. People may have a hard time focusing on work or other everyday activities. They may feel numb, anger easily, or cry often.

Priscilla Dass-Brailsford, a psychologist at Georgetown University Medical Center, said that if those symptoms don’t fade in about a month, of if they are bad enough to impair function, people should seek help.

But for most, “time is a great healer,” said Dass-Brailsford, who served on disaster mental health teams that counseled survivors of 9/11 in New York.

Specialists say only a small number of people are expected to be so severely affected that they develop PTSD, a disorder that can include flashbacks, debilitating anxiety, irritability and insomnia months after the trauma. Even among veterans of the Iraq and Afghanistan wars, the best estimate was that just under 20 percent returned with symptoms of PTSD or major depression.

More at risk for lingering psychological effects are people who’ve previously been exposed to trauma, whether from the battlefield, a car crash or a hurricane.

During two stints in Iraq as a Marine, Eusebio Collazo of Humble, Texas, was gravely wounded and today runs regularly to help deal with PTSD. Running with a veterans group called Team Red, White Blue, he was at mile 25 of the marathon when the bombs detonated – and adrenaline fueled his frantic race to find his wife, Karla, at the finish line. She was unharmed.

“My wife keeps asking me, `I don’t know how I should be feeling. I want to cry but I can’t.’ And then I want to cry, and I can’t cry either. So, there’s a lot of weird, different feelings going on,” Collazo said Thursday. It’s harder, he said, to handle explosions on the home front than in a war zone.

In Boston’s hospitals, teams of counselors and social workers are telling patients and their families what to expect in the difficult days and weeks ahead.

“Most people are having a lot of flashbacks,” and thoughts of the bombing interrupt their days and nights, said Lisa Allee, who directs the Community Violence Response Team at Boston Medical Center. “These are very typical, normal, expected emotions after any traumatic event or disaster.”

Beyond hospitalized patients, part of coping is awareness about how to take care of the psyche – turning off scary TV coverage and reading a book, going out for a quiet dinner, anything to temporarily cut the stress, says Dass-Brailsford, the disaster specialist.

That’s especially true for parents who are trying to calm their children, added UCLA’s Mogil, because kids take their emotional cues from the adults around them. Younger children especially don’t need to see repeated footage of the blasts, because they may think it’s happening again.

For a lot of people, psychiatrists say, talking about their experience can be cathartic.

A cashier’s routine “how are you?” was enough for Anndee Hochman to tear up in a Philadelphia hardware store Wednesday. Hochman and her 12-year-old daughter had traveled to Boston to watch her partner run the marathon – and all three were in different places when the bombs exploded, Hochman herself just a few blocks from the finish line.

Hochman spent 10 minutes telling the store clerk her family’s story of reuniting – and said it helps every time she’s told friends, family, even a near-stranger about the experience.

Unknowingly, Hochman echoed the advice to look for a silver lining as she counseled daughter Sasha, who was nervous about returning to school.

“I reminded her, ” `Sweetie’ – and reminded myself, too – `there may have been a few people who planned those bombs and wanted to hurt people,” Hochman said, “but there are so many more people there and in the world who want to help.’”