Seven years ago, Kristie Tunick had a promising future. She was single, beautiful, healthy and active in sports and dance. Then her life got even better when she met Brian Mendenhall.

"He's drop-dead gorgeous," Tunick told her friend the night she met Mendenhall, when he was working at a bar at a nightclub in Las Vegas.

Mendenhall said he was equally attracted. Tunick "had this glow about her. Smile. Genuineness. And just made you feel good," he said.

Today the couple is not only still together, they are even engaged. Yet their fairytale story is tinged with sadness.

In 2006, when Tunick was only 29, her health started to deteriorate inexplicably. What began as stomach issues, led to the removal of her gallbladder and slowly snowballed into even more severe symptoms: The muscles in her body began to lock up, virtually paralyzing her for hours and causing excruciating pain. Sometimes, she would even stop breathing.

CLICK HERE to see more photos of Tunick before mysterious illness

In and out of hospitals many times, Tunick was a real-life medical mystery who stumped the doctors who tried to diagnose her.

Today, this once active girl is now a prisoner in her own bed, and it is Mendenhall who takes care of her.

"I've never loved anyone like I've loved her. ... I'm not the type of person to just close my eyes, turn my back, and walk out. ... So I'm here by her side ... no matter what," he told ABC News correspondent Bob Brown.

As the couple searches for a solid diagnosis and waits anxiously for a specialty clinic to admit Tunick, Mendenhall has had to watch his fiancee be robbed of the life they once knew.

"We liked to play tennis. We loved to golf. We golfed all the time," he said.

Tunick's autoimmune disease now not only leaves golf out of the question, but sometimes even walking to the bathroom is a herculean task for her.

To donate money or send messages of support, visit Kristie Tunick's Carepage or join her cause on Facebook

For more information on SPS from Johns Hopkins University, click here.

Mystery Ailment May Be Stiff Person Syndrome, but Diagnosis Remains Uncertain

"Every day is torture," she said, "Every day is -- I feel like I'm being tortured. I feel like this is prison. ... I'm a prisoner in my body. I'm a prisoner in bed."

Many of Tunick's symptoms are similar to those of a person with stiff person syndrome, or SPS. Dr. Beth Murinson, a leading expert in SPS at Johns Hopkins University Hospital, said the key features of this complex disease are "episodic muscle spasms and ... persistent stiffness."

But not everyone's symptoms are as debilitating as Tunick's. Some people living with SPS are "bedridden or in a wheelchair while others are out playing tennis," Murinson said.

While blood tests aid in diagnosing this disease, they are not definitive: The doctor needs to know the history and physical condition of the patient before making a firm conclusion.

Harder still is finding a doctor who is familiar enough with the syndrome to make a confident diagnosts. According to Murinson, fewer than five in a million people have SPS, which makes finding a doctor who specializes in this disease also a rarity.

"I've been denied by at least four neurologists in town because they all felt I needed to be at a specialty ... clinic," Tunick said. "They're too scared to touch me."

Tunick's mysterious ailment has forced her to refocus her priorities. When ABC News correspondent Bob Brown asked Tunick what her dream is, her answer was realistic and short term: "I need to get somewhere."

Mendenhall agrees. "All my attention and focus and -- goals right now are ... getting her somewhere."

Yet, just "getting somewhere" is not as easy as it sounds, and even a simple trip to the local doctor calls for painstaking preparation.

"We have to use my father's car. We pad the back of it ... for me to lay in the back. And it's very painful. And I get really stiff with the bumpiness and the padding," Tunick told ABC.

Loyal Fiance: 'I Made a Choice to Stick by Her Side'

Through it all, the couple tries to focus on each other and the positive aspects of their lives.

"I can't be depressed and down and negative, 'cause I know that's not gonna help her," Mendenhall said. "Even though it's hard."

Their relationship has significantly transformed with Tunick's decreasing health, but their dedication to each other remains. Because of Tunick's limitations, they spend most of their time together in bed.

"Just sitting next to each other is romantic. And being with each other, and just seeing him and smiling -- that's romance," Tunick said. "You don't need gifts and flowers ... you just need that mental connection."

Tunick keeps her yearly resolutions posted on the wall next to their bed, a daily reminder of her hopes and aspirations for the near future: "Get healthy enough to leave the condo. Get in shape. Eat healthy. Stay positive. Keep hope. Stay organized."

And lastly, and most importantly, "And always keep Brian as my first priority."

For Tunick, her medical future remains uncertain. Their love for each other, however, remains a comforting constant.

"I made a choice to stick by her side. And I am. And I will ... 'til the end," he said.

He said he wants Tunick to know "my life's fine. My life is great with you. My life is nothing without you. Remember that."

To donate money or send messages of support, visit Kristie Tunick's Carepage or join her cause on Facebook.

For more information on SPS from Johns Hopkins University, click here.