Justin Bieber -- His Shirtless Disease Is Spreading

It's worse than the plague ... Justin Bieber's shirtlessness has now spread to his best friends, and chances of survival are slim.

Bieber's pal Lil Twist -- the guy wearing the Jamiroquai hat -- posted the photo on Instagram yesterday, showing just how bad the pandemic has gotten.

We've spoken to several experts who say it's only a matter of time before it spreads to unsuspecting non-douchebags across the country. Board up your windows.

FYI, the photo is also significant because it shows Lil Twist and Bieber are back on good terms after Twist was accused of battering some girl at Bieber's pad earlier this month ... but who cares!!! THIS IS A MEDICAL EMERGENCY!!!