Cave fungus (Fallout 3)

 is a food item found in Fallout 3. It can restore some of your Hit Points while slightly reducing your radiation.

It can be acquired from Little Lamplight by striking a deal with the "cook" Eclair or the "police officer" Knock Knock for either Strange Meat or Buffout when you have the Child at Heart perk. This arrangement can be extremely profitable for you due to the high value of cave fungus.

According to Residents of Little Lamplight and its history, the fungus is somewhat connected to human meat, hence why it "likes" Strange Meat. The original fungus grew "out of the water where the first Little Lamplighters dump the bodies of the mungos".