Cave fungus (Fallout 3)

Cave fungus is a aid item  It can restore some of the player character's Hit Points while slightly reducing their radiation.

Gameplay
It can be acquired from Little Lamplight by striking a deal with Eclair or Lucy for either strange meat or Buffout respectively. Alternatively, it can be acquired directly through Mayor MacCready after a successful speech challenge, but this will result in negative Karma. The price can be lowered with a higher Barter skill or with the Child at Heart perk.

According to the residents of Little Lamplight and its history, the fungus appears to "like" strange meat. This is connected to the fact that the original fungus grew "from the water where the first Little Lamplighters dumped the bodies of the mungos".

The fungus' radiation scrubbing ability makes it an effective and nutritious health booster out in the wasteland.

Unused Vault 106 note
The following is from a cut Vault 106 note in Fallout 3. It describes fungus growing in caves.