Pulmonary

If a tuberculosis infection does become active, it most commonly involves the lungs (in about 90% of
cases).[10][52] Symptoms may include chest pain, a prolonged cough producing sputum which may be bloody,
tiredness, temperature, loss of appetite, wasting and general malaise.[10][53] In very rare cases, the infection
may erode into the pulmonary artery or a Rasmussen aneurysm, resulting in massive bleeding.[5][54]
Tuberculosis may cause extensive scarring of the lungs, which persists after successful treatment of the
disease. Survivors continue to experience chronic respiratory symptoms such as cough, sputum production,
and shortness of breath.[55][56]
Extrapulmonary
Main article: Extrapulmonary tuberculosis
In 15–20% of active cases, the infection spreads outside the lungs, causing other kinds of TB.[57] These are
collectively denoted as extrapulmonary tuberculosis.[58] Extrapulmonary TB occurs more commonly in people
with a weakened immune system and young children. In those with HIV, this occurs in more than 50% of
cases.[58] Notable extrapulmonary infection sites include the pleura (in tuberculous pleurisy), the central
nervous system (in tuberculous meningitis), the lymphatic system (in scrofula of the neck), the genitourinary
system (in urogenital tuberculosis), and the bones and joints (in Pott disease of the spine), among others. A
potentially more serious, widespread form of TB is called "disseminated tuberculosis"; it is also known
as miliary tuberculosis.[5] Miliary TB currently makes up about 10% of extrapulmonary cases.[59]
Symptoms of extrapulmonary TB usually include the general signs and symptoms as above, with additional
symptoms related to the part of the body which is affected.[60] Urogenital tuberculosis, however, typically
presents differently, as this manifestation most commonly appears decades after the resolution of pulmonary
symptoms. Most patients with chronic urogenital TB do not have pulmonary symptoms at the time of
diagnosis. Urogenital tuberculosis most commonly presents with urinary 'storage symptoms' such as
increased frequency and/or urgency of urination, flank pain, hematuria, and nonspecific symptoms such as
fever and malaise.[61]
Causes
Mycobacteria
Main article: Mycobacterium tuberculosis
 Scanning electron micrograph of M. tuberculosis
The main cause of TB is Mycobacterium tuberculosis (MTB), a small, aerobic,
nonmotile bacillus.[5] It divides every 16 to 20 hours, which is slow compared with other bacteria, which
usually divide in less than an hour.[62] Mycobacteria have a complex, lipid-rich cell envelope, with the high
lipid content of the outer membrane acting as a robust barrier contributing to their drug resistance.[63][64] If
a Gram stain is performed, MTB either stains very weakly "Gram-positive" or does not retain dye as a result
of the high lipid and mycolic acid content of its cell wall.[65] MTB can withstand weak disinfectants and survive
in a dry state for weeks. In nature, the bacterium can grow only within the cells of a host organism, but M.
tuberculosis can be cultured in the laboratory.[66]
The term M. tuberculosis complex describes a genetically related group of Mycobacterium species that can
cause tuberculosis in humans or other animals. It includes four other TB-causing mycobacteria: M. bovis, M.
africanum, M. canettii, and M. microti.[67] M. bovis causes bovine TB and was once a common cause of
human TB, but the introduction of pasteurized milk has almost eliminated this as a public health problem in
developed countries.[68][69] M. africanum is not widespread, but it is a significant cause of human TB in parts
of Africa.[70][71] M. canettii is rare and seems to be limited to the Horn of Africa, although a few cases have
been seen in African emigrants.[72][73] M. microti appears to have a natural reservoir in small rodents such as
mice and voles, but can infect larger mammals. It is rare in humans and is seen almost only in
immunodeficient people, although its prevalence may be significantly underestimated.[74][75]
There are other known mycobacteria which cause lung disease resembling TB. M. avium complex is an
environmental microorganism found in soil and water sources worldwide, which tends to present as
an opportunistic infection in immunocompromised people.[76][77] The natural reservoir of M. kansasii is
unknown, but it has been found in tap water; it is most likely to infect humans with lung disease or who
smoke.[78] These two species are classified as "nontuberculous mycobacteria".[79]
 Public health campaigns in the 1920s tried to halt the spread of
TB.
Transmission
Tuberculosis spreads through the air when people with active pulmonary TB cough, sneeze, speak, or sing,
releasing tiny airborne droplets containing the bacteria. Anyone nearby can breathe in these droplets and
become infected. The droplets can remain airborne and infective for several hours, and are more likely to
persist in poorly ventilated areas.[80]
Risk factors
Main article: Risk factors for tuberculosis
Risk factors for TB include exposure to droplets from people with active TB and environmental-related and
health-condition related factors that decrease a person's immune system response such as HIV or taking
immunosuppressant medications.[81]
Close contact
Prolonged, frequent, or close contact with people who have active TB is a high high risk factor for becoming
infected; this group includes health care workers and children where a family member is
infected.[82][83] Transmission is most likely to occur from only people with active TB – those with latent
infection are not thought to be contagious.[68] Environmental risk factors which put a person at closer contact
with infective droplets from a person infected with TB are overcrowding, poor ventilation, or close proximity to
a potentially infective person.[84][85]
Immunodeficiencies
The most important risk factor globally for developing active TB is concurrent human immunodeficiency virus
(HIV) infection; in 2023, 6.1% of those becoming infected with TB were also infected with HIV.[15] Sub-
Saharan Africa has a particularly high burden of HIV-associated TB.[1] Of those without HIV infection who are
infected with tuberculosis, about 5–15% develop active disease during their lifetimes;[51] in contrast, 30% of
those co-infected with HIV develop the active disease.[50] People living with HIV are estimated 16 times more
likely to fall ill with TB than people without HIV; TB is the leading cause of death among people with HIV. [1]