Not all get ill, even if they have been in contact with a person who has infectious tuberculosis.
Tuberculosis (TB) is not easily transmitted. Only one third of those who have been in close contact over time with a person with infectious TB become infected.
The majority, nine out of ten, of infected persons never develop active TB disease. TB bacteria remains sleeping in their body (latent tuberculosis infection= LTBI). They do not have symptoms, they are not infectious and cannot spread the bacteria to others. However, if TB bacteria become active in the body and multiply, the person will go from having latent TB infection to being sick with TB disease.
People with TB disease are sick. The disease may be spread to those who spend much time with the sick person.
Only one in ten healthy adults infected with TB develops TB disease. About half of them get symptoms within two years. The second half will develop disease sometime later in life.
Small children, elderly and those whose immune system is weakened due to disease or medication have increased risk of developing TB disease.