Cowboys player Travis Frederick diagnosed with Guillain-Barré syndrome. What is it, exactly?

Dallas Cowboys offensive lineman Travis Frederick has been sidelined for the past few weeks during the team’s training camp due to health issues, and now he has a diagnosis. Frederick revealed on Twitter on Wednesday that he has Guillain-Barré syndrome, an autoimmune disease.

Frederick said was diagnosed after “a very extensive examination and discovery process.” He has received two treatments to address his condition over the past two days. “I am feeling much better from an overall strength perspective,” he added. “I will continue these treatments over the next few days. I am very optimistic about my condition and the immediate future, as I have been told that the illness was detected at a fairly early stage.”

Guillain-Barré syndrome (GBS) is a rare neurological disorder in which the body’s immune system mistakenly attacks part of its peripheral nervous system, the network of nerves located outside of the brain and spinal cord, according to the National Institute of Neurological Disorders and Stroke (NINDS).

The symptoms can range in severity from brief weakness to paralysis that can leave a person unable to breathe without assistance, the NINDS says. People typically have tingling hands and feet or pain in the legs or back, as well as weakness on both sides of the body, the NINDS says.

“You start to see weakness in your limbs, usually in the legs, that then goes up to the arms,” infectious disease expert Amesh A. Adalja, MD, senior scholar at the Johns Hopkins Center for Health Security, tells Yahoo Lifestyle. “It’s gradual and comes on over time, but it marches up your body.”

GBS is rare — it affects just one in 100,000 people each year, according to NINDS estimates.

Frederick didn’t reveal how he contracted GBS, if he knows at all, but people typically develop it after having an infection, Adalja says. “One of the most common inciting factors is an infection of some sort that starts an autoimmune process,” he says. It can be a food-borne infection, like Campylobacteriosis, or a virus like Zika, or even the flu, Adalja says. “Technically, any infection can cause it, but we don’t know why some people get it after an infection and others don’t,” he says.

There’s no single cure for GBS, but the disease is usually treated with plasma exchange, which involves extracting plasma from blood before returning it to the patient, or high-dose immunoglobulin therapy, which is an injection of proteins that the immune system naturally makes to attack infecting organisms, Adalja says.

“Over time, people do see improvement, but sometimes they can be left with permanent deficits,” Adalja says. Some people may have to undergo physical therapy, depending on how severe their illness was. Luckily, 70 percent of people with GBS eventually have a full recovery, the NINDS says.

While Frederick’s doctors have told him “that it is not possible to determine a time table for a return to the field right now,” he said he’s hopeful that he’ll “be able to play as soon as possible.”

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