How did they narrow it down to the gallbladder back then?
Presumably correlation on soldiers. Medicine was kind of hit or miss back then but quite often got lots of data from war wounded.
Typhoid had a lot of attention because it tended to be one of those things that killed a LOT of soldiers. The vaccine for it was developed in 1896 by the Almroth Edward Wright at the British Army Medical School. And the Second Boer War was the first time that war casualties were actually larger than disease casualties because of the vaccine.
Even in someone who’s actively contagious?
Yes.
Quoting https://www.sciencedaily.com/releases/2010/02/100222162004.h...:
"In fact, the most widely accepted treatment of chronic typhoid infection is removal of the gallbladder."
And, in particular, it looks like the most chronic and asymptomatic (like Typhoid Mary) have biofilms on gallstones that serves as a very difficult to kill reservoir.
Um, it's not a theory--it really works most (>90%) of the time. The gallbladder is generally the site of the chronic salmonella infection that couldn't be treated before antibiotics.
Now, whether you want to have surgery back in a time before antibiotics is an independent problem.