Not everyone gets sick, even if they have been in contact with someone 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.
Not everyone who gets infected will develop TB disease.
The majority i.e. nine out of ten, of infected people never develop active TB disease. TB bacteria remain dormant in their body (latent tuberculosis infection= LTBI). They have no symptoms, are not infectious and cannot spread the bacteria to others.
However, if TB bacteria become active in the body and start to multiply, the TB infection becomes TB disease.
In this case the person is sick and the disease can spread to those who spend much time with the sick person.
Only one in ten healthy adults infected with TB develop TB disease. About half of them will develop symptoms within two years of infection. The other half will develop TB disease sometime later in life.
Small children, the elderly and those whose immune system is weakened due to disease or medication are at an increased risk of developing TB disease.