Lyme disease is a vector-borne disease, the most common one in the United States. It's typically spread when an infected, blacklegged tick bites a person. Early symptoms of Lyme include fever, headache, fatigue, and a skin rash called erythema migrans, most of which Justin appears to have had. If the disease hadn't been caught early on, it could have spread to the singer's joints, heart, and nervous system.

Justin called Lyme disease an "incurable illness" in his post and he's not exactly correct. If caught early on, Lyme disease can be cured with antibiotics. The issue is that, if it's not caught early on, there may be more permanent damage done, like how Kris Kristofferson's Lyme disease caused struggles with memory loss. 

Rarely, a Lyme disease sufferer will still have symptoms after they have been treated with antibiotics, which is known as post-Lyme disease syndrome. That is technically incurable as doctors are unaware of what causes it.

Chronic mononucleosis and lyme disease can actually present pretty similarly, which may explain why Justin's diagnosis was so difficult. Mono is spread through saliva and, although it is commonly referred to as "the kissing disease", Justin could have gotten it any number of ways, including sharing a drink with someone or even if someone coughed towards him.

Most cases of mono are caused by infection with the Epstein-Barr virus (EBV). Unfortunately for Justin, who likely had mono when he was younger, EBV will live in him, usually in a dormant state, for the rest of his life.

According to the Mayo Clinic, "Periodically, the virus may reactivate. When this happens, the virus can be detected in your saliva — but you're not likely to become ill. Rarely, reactivated EBV may cause illness in people who have weak immune systems." Unluckily for Justin, Lyme disease is believed to damage one's immune system.