How to reach Plane of Disease
The zone in for Plane of Disease is located in the northeastern portion of Plane of Tranquility, just north of where you zone in from PoK at. You'll notice this zone in since it sticks out quite a bit - just keep an eye out for the large poopy pipe. It's shown in the picture above if you don't know what I am talking about.
One you come across this pipe you'll need to jump in to gain access to the Plane of Disease. You'll need to be at least level 46 - no flags are required to zone in.
Still having trouble locating Plane of Disease? My map below shows the exact location of the zone in - just look for the small blue circle above my green arrow.
Once inside Plane of Disease it isn't initially obvious how to get out. If you're looking for the zone out, it's right around the area you zoned in at, just underground. You reach the zone line by clicking on a trap door located on the floor and fall through.