"If you've experienced this disease or know someone who has — then you're aware: living with this can be relentlessly debilitating, both mentally and physically," Timberlake wrote. "When I first got the diagnosis I was shocked for sure.

As pop star Justin Timberlake shares his diagnosis of Lyme disease, his ordeal brings up questions. What is Lyme disease and its cause? How is the illness treated? A doctor explains.

This undated provided by the U.S. Centers for Disease Control and Prevention shows a blacklegged tick, also known as a deer tick, a carrier of Lyme disease. Justin Timberlake announced this week that he's been diagnosed with Lyme disease.