Diana Gabaldon has outdone herself with this one. You see, as a member of the military, one of his duties is to inspect the troops for all manners of venereal diseases, so he knows the clap when he sees it. At that point in history there was no cure, and one would be discharged immediately for having it. Also, there was no proper way of addressing it in the society that Lord John belonged to without creating an unacceptable scandal for his cousin that would make her unmarriageable and ruin her reputation. These things just were not done at that time and to be avoided at all costs. Moreover, he worried that his even noticing such a thing would out him as a homosexual. This situation was highly entertaining and how it is worked out is very clever. Also, there is the matter of missing military documents that Lord John is charged with finding.
This is the first book in the Lord John series and Lord John is physically described as is Lavender House, and he meets his paramour that is in Brotherhood of the Blade. Also in this book he hires his valet, Tom Byrd.
I highly recommend this book as a historical mystery.