ELI5 Blood Oxygen - why are pulse oximeters suddenly so popular?

How do pulse oximeters work? Why have they become popular recently? Can they be used to detect Covid-19?   ... We explain like i'm five!   Thank you to the r/explainlikeimfive community as always and in particular the following users whose questions and comments formed the basis of this discussion: cutelyn22, internetboyfriend666, scubasteave2001, scarynut, bravo_bravos & rauillindion To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Send us an e-mail: [email protected]