[Question] How useful are masks during an epidemic?

This was a pretty con­tentious part of the de­bate on the CDC’s recom­men­da­tions and has come up el­se­where as well: how effec­tive are masks at pre­vent­ing in­fec­tion when worn by an un­in­fected in­di­vi­d­ual? How about pre­vent­ing spread when worn by an in­fected in­di­vi­d­ual? Are there things you can do that make it more or less use­ful?

This is a thread for ev­i­dence to an­swer those ques­tions and re­lated ones. Ob­vi­ously the more rigor­ous the bet­ter, but case stud­ies like this one about one dude on a bus would be use­ful had they not been re­tracted (thanks to gwillen for point­ing this out).