K'heltan was a Cystaron exile that fled to 26th century Earth to ensure his survival, by masquerading as a dying alien. His spores caused a rabid spread of a virus that sustained his own life.
Exiled from his homeworld, and injured severely during his crash into the Earth, K'heltan found the human race to be a valuable source of sustenance, by using "spores" to infect humanity and feed of their own life. Fearing reprisal, K'heltan pretended to be dying, and in doing so tricked the inhabitants of Colony UK to let him live in their settlement.
When Everett, the Colony UK's chief security officer, suspects K'heltan of malign intentions, the Cystaron kills him by infecting him directly with his disease. K'heltan likely killed several others who suspected him, before the TARDIS arrived at Colony UK and the Doctor began to investigate the disease.
When it is discovered K'heltan is responsible, the Doctor tries to reason with the Cystaron, offering him safe passage to his homeworld, K'heltan refuses, saying he is an exile. He then spreads his spores throughout Colony UK and the mortality rate on the colony rapidly increases. K'heltan is destroyed when intense heat incinerates him. (DW: Devour)