BBC: TB Killed off Leprosy.

Scientists at University College of London speculate that since historically TB started to increase about the time that Leprosy disappeared in Europe, that maybe it was because of cross immunity.
Now, I don't know much about leprosy, but if you scroll down about the way the body attacks TB, a similar T cell (if I remember correctly) hits the infection in leprosy.
Now, ancient leprosy may or may not have been Hanson's disease-- there is an ongoing argument that it may have been a disease called Yaws, which causes bubos and is a spirochete related to Syphillis. (There is another argument if Syphillis came back with Colombus, or if sailors caught yaws going around the horn of Africa and the disease changed---and syphillis changed after arriving in Europe, from "the great pox" to a slow disease that took years to kill).
Now, most cases of Hansen's disease is caused by being very dirty-- Father Damian caught it because there was no good water supply when he arrived in Molakai, and he was in close contact. However, few modern nurses have ever caught the disease, and indeed, the suspicion is that most cases are "caught" by infants or children who have thin skin and immature immune sytems, and smolder awhile before showing up in young aduts.

So although the TB causing cross immunity theory is nice, I posit two other explanations: One, a gradual increase in cleanliness and the standard of living. Two: People whose immune system was weak from Hanson's disease quickly died of TB, and never spread the disease to new cases.

BTW: Hanson's disease is fairly easily treated in villages, but leprosariums still exist because many cases are thrown out of families when discovered.

James Carville, call your office...