TB is primarily an airborne disease, which is spread from an individual with active disease in tiny droplets when a TB sufferer coughs, sneezes, speaks, sings or laughs.
Only people with active TB whose sputum contains M. tuberculosis are contagious. Lengthy contact with this type of patient is usually required before a person can become infected.
In most people infected with TB, the immune system fights the bacteria and can prevent them from multiplying.
Only approximately 10% of all infected people develop active disease, the majority within 2 years of exposure to M. tuberculosis.
In a substantial proportion of TB-infected individuals, the bacteria become inactive for a long time but remain alive in the body and can become active later in life as immunity decreases with age. This is called latent TB infection.
Risk factors for active TB include: poverty, malnutrition, poor housing conditions, inadequate healthcare systems, drug abuse (including alcohol) and old age.
Genetic susceptibility may play an important role in determining whether an infected individual develops active disease.
Patients infected with HIV have a much higher risk (up to 70-times) of becoming infected with M. tuberculosis and an up to 100-times higher risk of developing active TB than HIV-negative individuals.