Nancy Méndez-Booth was diagnosed with PTSD after she delivered a stillborn baby in 2008. Within an hour after she rushed to the hospital, in labor and exhilarated, a doctor told her that the baby she had spent years planning for had no heartbeat.

When she returned home, Méndez-Booth said she felt as if she had "arrived from Mars"; she got lost in her own apartment building. She oscillated between numbness, vivid paranoia — she worried the police would arrest her for her son's death — and bursts of anger. Her kitchen cabinets became loose because she would bang the doors together, over and over, looking for a way to let out some of her rage.

"I would just think to myself, Who in their right mind experiences four different, incredibly intense mental states in the span of 15 minutes?" said Méndez-Booth, a writer and educator in New Jersey. She couldn't differentiate between the past and the present; she kept flashing back to the delivery table. She thought she was experiencing a psychotic break, but later, she found out she was experiencing post-traumatic stress disorder, or PTSD.

Méndez-Booth counts herself as lucky to have gotten a diagnosis. According to experts, it is highly common for the disorder to go undetected. Misconceptions about who develops PTSD, and confusion over its complex cluster of symptoms, can prevent people with the disorder from seeking treatment — or realizing they have it at all. "You're talking about millions" of people suffering from PTSD without a diagnosis, said Bessel van der Kolk, author of the seminal book on the subject, "The Body Keeps the Score," and an expert in the field of treating trauma.

PTSD entered the Diagnostic and Statistical Manual of Mental Disorders in 1980 as an official diagnosis, in response to symptoms that Vietnam War veterans were exhibiting, and today, people in combat still report high rates of the disorder. According to the U.S. Department of Veterans Affairs, between 11 and 20% of veterans who served in Operations Iraqi Freedom and Enduring Freedom have PTSD in a given year.

But civilians are affected by post-traumatic stress, too.

The trauma most likely to cause PTSD is rape, with combat trauma as a "close second," said Dr. Shaili Jain, a PTSD specialist at Stanford University. That's why she, along with van der Kolk and other experts, say it is vital for more people to understand what PTSD really is.

Studies show that early intervention is critical for managing, and potentially preventing, PTSD. But it can take up to two years or more for people exhibiting symptoms to get a diagnosis, Jain said, and those who don't receive treatment within the first two years have much lower odds of recovery.

"Hearing I had PTSD — it felt like I didn't earn it," said Natalia Chung, 30, who was diagnosed with the disorder in 2016 after ending an abusive relationship. "Because I didn't go to war," she said.