Patient compliance is a major problem. But, how bacteria causes disease is not known. It is known, for example, that with cholera, the bacteria make toxins and the toxins cause disease. One theory of how TB causes disease is that the body's chronic response to TB is what causes disease. Host immune response can be damaging to host cells.
Gradual damage is due to chronic conditions.
There can be tissue damage, lesions, necrosis, and reparative fibrosis. When tissue is destroyed and is replaced by scar tissue, the airspace shrinks. TB does cross the blood-brain barrier. TB has very little evidence for DNA exchange. Macrophages absorb TB and the bacteria live inside the cell.