Tuberculosis (TB) is most commonly a respiratory disease, a disease of the lungs. It is spread through droplets in the air; someone with TB coughs or sneezes, for example, someone else breathes in the virus and the TB settles in the lungs.
Once the TB virus has settled in the lungs, it can travel to other parts of the body, through the blood, and settle in the bones. One thing to keep in mind is that TB in a bone is not usually contagious.
TB of the knee shows up as pain and swelling, making it hard to bear weight and resulting in a limp.