Disease is contagious and will infect other adjacent creatures of the same species.
Visually, a diseased character is easily recognised via the swirling green gases and flies surrounding him.
Disease turns the target's health bar green. In PvE, it can spread from human foes such as Corsairs or the Jade Brotherhood to players characters, heroes, and henchmen. In PvP, all characters and NPCs aside from pets, Juggernauts, and Turtles are considered to be the same type of creature.
If you cast a spell causing Disease as a necromancer and that creature spreads it back to you, using Plague Touch, Plague Signet, or Plague Sending while that creature is still close to you will not cause the Disease to immediately return to you, even though it meets the requirements of spreading.