"While a lot of people kept saying Justin Bieber looks like sh*t, on meth etc. they failed to realise I've been recently diagnosed with Lyme disease, not only that but had a serious case of chronic mono which affected my, skin, brain function, energy, and overall health," he wrote.
Justin went on to tell fans that he would go into further detail about his condition on an upcoming doco he's been working on.
"These things will be explained further in a docu series I'm putting on YouTube shortly.. you can learn all that I've been battling and OVERCOMING!! It's been a rough couple years but getting the right treatment that will help treat this so far incurable disease and I will be back and better than ever NO CAP," he wrote on Instagram.
Glandular fever - also known as kissing disease - is an infection caused by the Epstein-Barr virus, and causes sufferers to feel run down and fatigued.
Lyme disease's most common symptoms are fever, headache, fatigue and a skin rash called erythema migrans, and it is usually contracted by a tick bite.
The 'Sorry' star allegedly said he had felt severely depressed for much of 2019, but it wasn't until later in the year that his doctors realised he was suffering from Lyme disease, sources who previewed the documentary tell TMZ.
While it's unclear how Justin contracted Lyme disease, a number of other celebrities have spoken about also being infected, including Bella Hadid, Shania Twain and Avril Lavigne.