ThePostGame.com Shannon Boxx

Shannon Boxx, one of the most decorated players in the history of U.S. soccer, played much of her career with a secret.

Boxx suffered from lupus, a chronic inflammatory disease that causes the body's immune system to become hyperactive and attack its own healthy organs and tissues.

But she didn't tell anyone that she started feeling symptoms as far back as 2002, when she felt extreme fatigue both on the soccer field and while doing simple tasks, like walking up stairs. And after being diagnosed in 2008, Boxx shared the news only with her coach and a teammate.

"When I kept it quiet, it was lonely," Boxx said.

Shannon Boxx

Boxx went public with her condition in advance of the 2012 Olympics where she helped the USWNT win a gold medal for the third time. The defensive midfielder continued to play until retiring at 38 in 2015, a few months after the U.S. won the Women's World Cup. In addition to the Olympics, she had four top-three performances in the World Cup, was a finalist for the 2005 FIFA World Player of the Year award and won a NCAA championship at Notre Dame.

But the unpredictability of Lupus and its symptoms made it difficult for Boxx to play soccer at an elite level.

"I remember having bad days, and I remember having to tell myself, 'Wake up tomorrow, tomorrow's a new day and we'll try again,'" said Boxx, whose symptoms also included joint pain.

Lupus is difficult to diagnose for many reasons, one being that there are no specific blood tests for it. Boxx saw multiple doctors for six years before getting a proper diagnosis.

"Symptoms are very non-specific at times," said Dr. Tania González-Rivera, medical director of Immuno-Inflammation at GlaxoSmithKline. "Patients can have joint pain and fatigue, but also they can have rashes, they can have a sensitivity to the sun or hair loss. Some of the symptoms can be seen in other diseases. It's sometimes called the Great Mimicker."

Shannon Boxx

Since disclosing her struggle in 2012, which gave her added strength and confidence in battling it, Boxx has been working hard to raise awareness about lupus and helping people to understand the symptoms. (May is Lupus Awareness Month.) She has been active with the Lupus Foundation of America as part of the mission toward finding a cure.

"The biggest thing is surrounding myself with support," Boxx said. "I didn't do it for so long on the national team, and when I did, there was such a big difference, a big relief."

Boxx says she is in remission but still has bad days. She is also busy as the mom to a 3-year-old and 6-month-old.

"They've made me realize that it's more important now, more than ever, that I make sure I take care of myself, for them," Boxx said.

She listens to her body carefully and keeps open communication with her family and doctor about her symptoms. Boxx and González-Rivera both recommend UsinLupus.com for resources to understand the disease and be better equipped to face Lupus.

"I wish it existed at that time when I was first being diagnosed because it's a place that you can go to track your symptoms," Boxx said. "You can keep a journal of all your notes for your doctor."

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