Singer Justin Timberlake has Lyme disease, a condition he says caused nerve pain, fatigue and other symptoms during his recent tour.

In a post on Instagram Thursday, the 44-year-old pop star said the tick-borne illness left him feeling “relentlessly debilitated, both mentally and physically.”

"When I first got the diagnosis I was shocked for sure. But, at least I could understand why I would be onstage and in a massive amount of nerve pain or, just feeling crazy fatigue or sickness," the singer wrote. 

Lyme disease is caused by bacteria spread through the bite of infected ticks. Symptoms include rash, fever, headache, extreme fatigue, stiffness in joints, muscle aches or weakness, according to the Mayo Clinic.

If left untreated, the disease can cause serious complications, including arthritis. Early treatment with antibiotics is key to stopping the infection from getting worse.

Timberlake did not share when he was officially diagnosed, but said the illness made him consider stepping away from performing.

"I was faced with a personal decision. Stop touring? Or, keep going and figure it out," the singer wrote. "I decided the joy that performing brings me far outweighs the fleeting stress my body was feeling. I’m so glad I kept going."

Online videos from the tour show the singer taking long breaks during songs and letting the audience sing for him. Some fans had criticized the shows.

Timberlake said he was hesitant to go public with his diagnosis but decided it was time to be “more transparent about my struggles so that they aren't misinterpreted.”