Tuberculosis transmission can only occur in people who have active disease. TB is transmitted through particles expelled by the patient smear-positive (with active TB) with coughing, sneezing, talking, singing, etc. Infectious droplets are of a diameter between 0.5 to 5µm and can be produced around 400,000 with a single sneeze. Each of these droplets from an active patient can transmit the organism or organisms, especially knowing that the infective dose of tuberculosis is considerably low, so that a single inhalation of the bacteria can cause an infection.
The likelihood of an efficient transmission increases with the number of contaminated particles expelled by the patient in how good is the ventilation of the area, duration of exposure and the virulence of the strain of Mycobacterium tuberculosis. People with frequent, prolonged or intense are at a 25% risk of being infected.
One patient with untreated active TB can infect between 10-15 people per year. Other risks include those areas where TB is common in immuno compromised patients with conditions such as malnutrition and AIDS, ethnic populations at high risk and health care workers serving high-risk regions. In patients with AIDS-TB, it acts as an opportunistic disease (infection) that is strongly associated. It can also be transmitted by the digestive route, mainly by drinking non-sanitized milk from cows infected with Mycobacterium tuberculosis and Mycobacterium bovis.
The chain of transmission can be broken if the patient is isolated with active tuberculosis and immediately starting effective anti-tuberculosis therapy. After two weeks with this treatment, patients with active TB and non-resistant TB will no longer be contagious. If a person were to become infected, it takes less than 21 days to one month before it can begin transmitting the disease to others.
Tuberculosis infection will progress to tuberculosis disease. It can occur at early stage (primary TB, about 1-5%) or after many years of infection (post-tuberculosis, secondary reactivation tuberculosis in about 5-9%). The risk of reactivation is increased with alterations in the immune system, such as those caused by HIV. In patients co infected with HIV and TB, the risk of reactivation increases to 10% per year, while in an immunocompetent person the risk is 5 to 10% over a lifetime.
Some drugs, including treatments currently used in rheumatoid arthritis that act by blocking the tumor necrosis factor, increase the risk of activating a latent TB because of the important action of this cytokine in the immune response against TB.