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PHILADELPHIA (CBS) – A mystery disease that’s difficult to diagnose and sometimes leaves patients feeling like they’re crazy because they’re always sick.

Emily Thomas went through most of her life feeling chronically ill.

“I remember being sick all the time, sinus infection, upper respiratory bronchitis,” said Thomas.

She battled Rheumatoid Arthritis and mono for months. Antibiotics did little to help. Doctors didn’t know what was wrong.

“There was definitely a point when I thought I was crazy,” said Thomas.

Dr. Rene Leon is an allergist and clinical immunologist who now treats Emily.

“Many of them think they are crazy because they are always sick,” said Dr. Leon.

Emily was diagnosed with common Variable immune deficiency, (CVID).

They call this the most common, uncommon condition.

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CVID impairs the immune system, patients have fewer antibodies to fight infection.

In addition to frequent bacterial infections, they may also have digestive issues, and chronic lung disease.

“When you do give them a diagnosis, instead of being sad, they are actually rejoiced because there’s an explanation as to why they are sick for the last 10 to 15 years,” said Dr. Leon.

People with CVID also have a greater than normal risk of developing certain types of cancers.

Emily battled an extremely rare childhood cancer as an adult. She believes the CVID is the common thread behind all of her illnesses.

Now, with an accurate diagnosis, she’s finally getting her life back.

“I’m dramatically better,” said Thomas.

She now gets at home IV infusions which essentially give her a boost of antibodies to fight infection.

“It’s your body, it’s your business and if you have a sense it’s not right, get a second opinion, get a third opinion,” said Thomas.

When patients are not diagnosed as children it can take 15 to 20 years for them to get real answers and treatment as adults.

Diagnosis generally involves simple blood work.