In high-risk groups TB disease is more common than in the general population. These high-risk groups can be divided into two categories:
- people who have a higher risk in getting exposed to TB and getting infected with TB and
- people who have a higher risk for developing TB disease once they have been infected
The risk of TB transmission is affected by the location and severity of TB disease, characteristics of the environment and the length of time the person is exposed.
Most commonly TB is spread by those who have infectious TB of the lungs because they have large quantities of TB bacteria in their lungs. When the sick person coughs or speaks, TB bacteria can get into the air. When other people stay in this same indoor environment, they are exposed to TB. They may breathe in bacteria in the air and get infected. This is more likely to happen if they stay there for a long time or often.
TB transmission is more likely, if the room is small and crowded and does not have adequate ventilation. In such settings the concentration of bacteria in the air can be high. For example transmission is more likely in a shelter for the homeless where many people sleep side by side in the same small room where the ventilation is poor, than in a shelter where only two people share a normal sized room where the windows are opened regularly.
Tuberculosis is not transmitted easily. Only one third of those who are exposed to TB get infected. Only one in ten adults with a normal immune system, infected with TB develops TB disease. About half of them will develop the disease within two years.
Some groups have a higher risk of developing the disease. Certain conditions, diseases and medications weaken the immune system which increases the risk of developing TB disease. They include: HIV-infection, diabetes, rheumatoid arthritis, severe kidney failure, leukemia, lymphoma, lung cancer, head and neck cancer and silicosis.
In Finland, the current risk groups for TB are:
- Close contacts of persons with infectious/ smear positive TB of the lungs: people living in the same household, relatives and other people ( coworkers, friends) who spend much time with the sick person. Exposed small children have a high risk of developing TB disease.
- Elderly people
- Substance (drug and alcohol) abusers and socially marginalized persons
- Immigrants from countries where TB is a common disease
- Others who have for a long time or frequently been (worked, studied or lived) in a country where TB is common
- People who have a condition, disease or medication which weakens the immune system, especially HIV-infection
- People who are exposed to TB at work