This disease is typically caused by sleeping with a person housing a certain bug found in most countries except vietnam. The bug is called a "urethratinsite". It crawls into the urethra and sucks all of the moisture out of the flesh and muscle in one swoop, leading the structure to dismember into a cloud of dust. The poof sound is created by the dust re-hydrating in the air.
*Please note, if a wang gets penisitis, it is then classed as being a peenie, not a wang* (Please refer to PEENIE)
"Dude, you might wanna go see a doctor about that, it looks like you got penisitis"