If you're an anthropomorphized pathogen, you do not want to kill your host. Ideally you don't even want your host to get sick. You want your host to be able to

If you're an anthropomorphized pathogen, you do not want to kill your host. Ideally you don't even want your host to get sick. You want your host to be able to survive and spread your descendants as widely as possible. This is why a lot of fictional descriptions of pandemics are unrealistic—something with a severe lethality rate has a hard time spreading (see, for example, Ebola which is highly contagious and fatal, but tends not to spread far from its animal reservoirs). With TB what happens is that the disease enters into an equilibrium state where the pathogen is able to survive in the host for years. There's been a lot of adaptation on both sides. There's still debate about whether TB was introduced to the Americas by Europeans or not, but it does have a disproportionately bad impact on Native American populations that lead me (uneducatedly) to think that it likely was introduced.