The patient with an active pulmonary tuberculosis infection is extremely contagious and should be isolated from family, friends, and the general population until the patient's physician and infectious disease specialist are certain that the infection is properly treated with a combination of sensitive antituberculosis medications.  When the patient with an active tuberculosis infection coughs, sneezes, talks, laughs, spits sputum, etc., he throws out into the air tiny droplets of sputum laden with tubercle bacilli. Any person, in close proximity to the infected patient is likely to inhale these tiny droplets. Many of these large size droplets fall on the ground, and may be inhaled later when they become dried out.  They may survive in the soil for as long as six months. Hence, even dust contaminated with tiny dried droplets of tubercle bacilli play a role in the transmission of the disease. Even the area or room, where a patient with tuberculosis has been isolated, remains potentially contaminated, as tiny droplets of the tuberculosis bacilli remain suspended in the air.

Eventually, the tubercle bacilli-laden sputum, in tiny droplets, enter through the nasal passages and finally reach the tiny air sacs of the lungs (alveoli) causing the primary pulmonary infection. 

Primary complex

During the initial/primary infection, the tubercle bacilli start multiplying very slowly in the body. Some remain in the air sacs (alveoli), while others enter the nearby regional hilar or mediastinal lymph nodes. Provided the natural defense mechanisms of the body are good, the lesions, as a result of invasion of tubercle bacilli, both in the air sacs (alveoli) or lungs, and in the glands, heal and become calcified. These healed / calcified lesions, i.e., in the lung, as well as in the glands, together, are called a primary complex. The patient will have hardly any symptoms during this phase of infection. It is of interest to the readers that this primary complex is supposed to safeguard the person against future infection of tubercle bacilli.

All persons in whom a primary complex has developed will be 'tuberculin skin test' positive, as the individual concerned would have developed hypersensitivity to the proteins of tubercle bacilli, known as 'tuberculins', which are used to perform this skin test. It is an important test, primarily meant to detect initial infection or a primary lesion/complex in an individual. This test has particular value in an epidemiology studies to find out the prevalence of tuberculosis infections in a community, i.e., 'tuberculin test surveys'.

In the case that the natural defense mechanisms of the body are not working properly, such as in the immunocompromised patient,  the primary / initial invasion by tubercle bacilli may result in active pulmonary tuberculosis.

Sites of infection of tubercle bacilli

However, in some cases, following primary infection by tubercle bacilli, the bacilli may remain dormant in the body indefinitely.  This is the most dangerous situation, since the tubercle bacilli may become active at any time, even after many years, and start multiplying and triggering the active disease. They truly act like potential enemies in the body, and attack whenever the defense mechanism of the body is suppressed. 

Dormant tubercle bacilli