Deathworm Morphant

Deathworms were symbiotic creatures used by the Morgs to enable them to survive beyond death.

Overview
When a deathworm was consumed, it would incubate in its host's body until the moment of death, then leave with the host's consciousness preserved inside it and seek out a new host. It could survive most form of abuse, including disintegration. The Master sought out the Morg to obtain a deathworm from them and consumed it to survive his execution on Skaro by the Daleks. (EDA: The Eight Doctors)

Afterwards, he managed to interferre in The Doctor's TARDIS to send it to the 30th December 1996. With the form of the Deathworm, the Master was able to slither out of the TARDIS via its keyhole and across the pavement, the Doctor saw this but he was unable to warn Chang Lee. Later that night the Master infiltrated the house of Bruce, the paramedic who attended to the wounded Doctor. He slithered down his mouth while he was asleep and possessed him, gaining a new body. (DW: Doctor Who)
 * Another account (DWM: The Fallen) claims that the Master used a morphant from Skaro to effect this.