"Shun them not only in reality, but even in appearance, if you would be well received in good company; for people will always be shy of receiving a man who comes from a place where the plague rages, let him look ever so healthy."
From LETTERS TO HIS SON By the EARL OF CHESTERFIELD on the Fine Art of becoming a MAN OF THE WORLD and a GENTLEMAN
Semantically, the highlighted part looks like a verb phrase with the subject being plague, I guess, but in that case, the verb let seems to be missing -s and a coordinator.
Please elaborate me about the status of the part.