tuberculosis, or tb, is one of the oldestand most common infectious diseases. about one third of the world population is believedto be infected with tb. fortunately, only about 5% of these infections progress to activedisease. the other 95% of infected people are said to have a dormant or latent infection;they do not develop any symptoms, and do not transmit the disease.tuberculosis is caused by a rod-shaped bacterium, or a bacillus, called mycobacterium tuberculosis.an infection is initiated following inhalation of mycobacteria present in aerosol dropletsdischarged into the atmosphere by a person with an active infection. the transmissionprocess is very efficient as these droplets can persist in the atmosphere for severalhours and the infectious dose is very low
â€“ less than 10 bacilli are needed to startthe infection. once in the lung, the bacteria meet with thebodyâ€™s first-line defense - the alveolar macrophages. the bacteria are ingested bythe macrophages but manage to survive inside. internalization of the bacilli triggers aninflammatory response that brings other defensive cells to the area. together, these cells forma mass of tissue, called a granuloma, characteristic of the disease.in its early stage, the granuloma has a core of infected macrophages enclosed by othercells of the immune system. as cellular immunity develops, macrophages loaded with bacteriaare killed, resulting in the formation of the caseous center of the granuloma. the bacteriabecome dormant but may remain alive for decades.
this enclosed infection is referred to aslatent tuberculosis and may persist throughout a person's life without causing any symptoms.the strength of the bodyâ€™s immune response determines whether an infection is arrestedhere or progresses to the next stage. in healthy people, the infection may be stopped permanentlyat this point. the granulomas subsequently heal, leaving small calcified lesions. onthe other hand, if the immune system is compromised by immunosuppressive drugs, hiv infections,malnutrition, aging, or other factors, the bacteria can be re-activated, replicate, escapefrom the granuloma and spread to other parts of the lungs causing active pulmonary tuberculosis.this reactivation may occur months or even years after the initial infection.in some cases, the bacteria may also spread
to other organs of the body via the lymphaticsystem or the bloodstream. this widespread form of tb disease, called disseminated tbor miliary tb, occurs most commonly in the very young, the very old and those with hivinfections. tuberculosis is generally treatable with antibiotics.several antibiotics are usually prescribed for many months due to the slow growth rateof the bacteria. itâ€™s very important that the patients complete the course of the treatmentto prevent development of drug-resistant bacteria and re-occurrence of the disease.