The Healer was an Immunoglobulin general who fought in the constant war against the Macro-viruses on Mimosa 3. During one confrontation with the viruses, they suffered many casualties and it was one of the very few left. The fear that afflicted it inhibited its abilities, and so it fled the planet.
The creature found its way to Earth and hid. It was found by Father Chadwick Vita, who thought it an angel and took it into his care. It healed the sick people Father Vita brought to him and gained a reputation as a faith healer.
At some point in 1669 it sensed its enemy nearby, but stayed at the church and continued to heal the sick at Father Vita's request. The Doctor came to the church while investigating the apparent outbreak of the Black Death, and immediately realised the healer's origins. He was soon followed by the Macro-viruses, who attacked the Church to destroy him. The Doctor urged the healer to be brave again to save Father Vita and the others and its powers were restored. It immediately divided and formed an army, which destroyed the viruses. Father Vita asked it to stay with him, but the healer left to fight its war.