Tuberculosis (TB) is an infection caused by the bacteria Mycobacterium tuberculosis (1) and remains the leading infectious cause of death worldwide (2). There is clear evidence that incidence of TB is increasing. It's estimated that between 2000 and 2020, nearly 1 billion people will be newly infected with TB, 200 million people will develop the disease, and 35 million will die from TB (3).
Mycobacterium tuberculosis (shown in red)
The process of catching tuberculosis involves two stages: first, a person has to become infected; second, the infection has to progress to disease (13).
Person can become infected and develop TB after inhaling droplets sprayed into the air from a cough, sneeze, or even talk by someone infected with Mycobacterium tuberculosis (4). A less common route of transmission is through the gastrointestinal tract by drinking milk infected with the tubercle bacillus (5) or skin wounds (e.g. pathologists and laboratory technicians who handle TB specimens) (11). Tuberculosis can only be spread primarily from person to person during close contact by breathing infected air (8). It cannot be spread through shaking hands or touching someone with the infection (9).
The likelihood of TB acquiring increases with:
1. The infectiousness of the person with TB (how many organisms are expelled into the air) (10). The infectiousness of a TB patient is directly related to the number of tubercle bacilli that he/she expels into the air. Patients who expel many tubercle bacilli are more infectious than patients who expel few or no bacilli. Patients are more likely to be infectious if they: have TB of the lungs or larynx, have a cavity in the lung, are coughing or undergoing cough-inducing procedures, are not covering their mouth when coughing, have acid-fast bacilli on the sputum smear, or are not receiving adequate treatment (17)
2. The environment in which exposure occurred. The risk of contracting TB increases with the crowded or unsanitary living conditions (6).
3. The duration of exposure. The incidence of TB increases with the frequency of contact people who have the disease (6).
4. The virulence (extreme harmfulness) of the organism (10).
Speaking, coughing and sneezing by TB infected people produces droplet nuclei containing infectious organisms, which can remain in the air for several hours (5) and can be inhaled through the nose and mouth by uninfected persons. Once the bacteria are inhaled they are engulfed by macrophages (white blood cells) that are present in the alveoli (the air sacs of the lungs) (15). From alveoli the bacteria spread through the lymph vessels to nearby lymph nodes (12) where it replicates within the macrophages for 2 to 3 weeks before spreading throughout the body (15).
The usual site of the disease is the lungs (Pulmonary Tuberculosis).
Tuberculosis in the lung Pulmonary TB