Now again thanks to Montague for introducing me to a new idea. I had long accepted the Halo as an artistic convention used to highlight the divinity of certain personages.
Not so, the Halo was first attempted to illustrate the irradiation that was said to accompany people of great divinity - Saints were said to glow with a great light - and the conventions of artists were attempts at replicating this.
He then discusses the bodies of Saints which were recorded as emitting light, which I find a lot easier to believe, it might be rare but the right combination of fungi and decomposition could conceivably make a corpse luminous.
Oddly enough, Google seems to turn up results that suggest it might not be impossible: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0006256 && https://www.ncbi.nlm.nih.gov/pubmed/15947465. A long way from a Halo, but that makes 2 things I didn't know. Curious and curiouser...