This is all vanilla gameplay. You can´t turn diseases off. (Maybe there´s a mod that can but I haven´t heard of it.)
I´m pretty sure your Bannis didn´t all die of tuberculosis, even if it´s a pretty nasty thing but as you´ve seen when you don´t have a doctor (and there is none you can build in an early Nordic game) they will stay at the doorway and not work until they are well again and of course, if it´s harvest time, it might turn out really bad.
The healer´s hut is no doctor, it´s a herbalist. Herbs can improve health if you don´t have a good diet (increase the hearts). If people are healthy it´s less probable that they get a disease but they may get ill also with 5 hearts.
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