NOTE: Patient Zero is a client-side event, which means that if you see patient zero and you are with a group, only you can see it and it will only affect you.
Patient Zero (also known as Subject Zero, Test Subject Zero, P-0 or Patient X) was the first infected citizen and the one who started the Apocalypse. Patient Zero is black with 4 tentacles on his back. He has no face and is often mistaken for Slenderman. Patient Zero is genderless, but is referred to as a 'he' in messages written in blood. Nobody knows his name, where he lived and who he is. He was taken away and experimented on by Trinity Corp. He has written messages in blood all over the map (see "Patient Zero's Messages"). When encountered, he will send you to the Kin Hospital with 1 Health left whether you had full health or not when you encountered him. Also Patient Zero was once a decal in the earlier versions. Kin Hospital has the most amount of messages related to the story of Patient Zero.
There's no way to " find " Patient 0, he spawns randomly around the map, sometimes in Union Square, and it's a rare event.