Well, I've used a heavily-modified version of the Phantom Fungus.
Did any of you ever see the episode of ST: TNG with the photomemetic chameleon creatures that reproduce by infecting other organisms? Geordi nearly became one permanently?
Well, that was basically what I did with the Phantom Fungus. My version was a predator that hunted small animals for food. Occasionally, it would try to ambush larger creatures, smearing some of the slime that covered its body onto them. Sometimes the creature wouldn't even notice. When it was attacked and killed, the fungus exploded in a shower of the same viscous goo. If the slime entered a break in the skin, the victim would slowly become a living, breathing host for the fungus. Plus the stuff was invisible, so it was difficult to know whether any had gotten on you and even harder to remove.
I just didn't think the standard version was scary enough.