Shadow Typhus

This magical illness infects a victim’s shadow, causing the victim to slowly become a dark shade.
  This diseases may be contracted if an infected creature stands in your shadow, or by lurking in the same shadow as an infected creature. Creatures of the outsider and aberration type are immune to the effects of the disease, but may still carry and transmit the disease to others.
  Symptoms develop the sunset after contraction.
  At that time, the victim begins feeling weak, and their shadow appears darker than usual. The victim begins to fear any light and feels pain and anguish if exposed to sunlight (this exposure does not cause any physical harm). The creature must make a DC 15 Constitution check to willingly step into any bright light.
  If allowed to linger in the shadows, the disease worsens, and the infected creature begins to look as though they are in the shadows even when in the light. After three days without exposure to any bright light, the creature begins a transformation into shadow that takes place from sunset to dawn after the third day of prolonged darkness. The infected creature becomes a shadow that is able to create other shadows through disease rather than by its strength drain ability (which otherwise remains unchanged).
  The only cure for this disease is prolonged exposure to sunlight (1 uninterrupted hour), radiant damage (equal to half of maximum hit points), or a lesser restoration. Spells and abilities that remove curses or diseases will also end this disease.