Tuberculosis (TB) is a mycobacterial infection most commonly caused by Mycobacterium tuberculosis. TB is distributed worldwide, and it is estimated that one-third of the world's population is infected (most without apparent disease). The majority of incidence cases come from India, Indonesia, China, Nigeria, Pakistan, and South Africa. Global incidence is on the decline since 2014, yet TB remains a top 10 cause of death worldwide. Risk factors for TB include poverty, being undomiciled, unemployment, crowding, and HIV infection. Most morbidity and mortality is seen in developing countries in tropical climates and in HIV-infected individuals. The risk of active TB in HIV-infected patients is 3%-8% per year with a lifetime risk of over 50%.

Cutaneous tuberculosis (CTB) is caused by M tuberculosis, Mycobacterium bovis, and the Bacillus Calmette–Guérin (BCG) vaccine. It is exceedingly rare, only occurring in 1%-2% of extrapulmonary TB cases. CTB is more common in children, pregnant individuals, and HIV-infected persons. Cutaneous manifestations of TB can be subclassified into 4 categories.

Exogenous Sources: Primary inoculation TB occurs at sites of trauma, abrasions, or wounds or at mucous membranes.

• In patients with no immunity against TB, a scab or chancre forms at the wound site and can be associated with regional lymphadenopathy after several weeks. Lymph nodes can eventually suppurate, abscesses can form, and fever and pain can develop. 
• In previously sensitized individuals, primary inoculation can lead to tuberculosis verrucosa cutis (warty tuberculosis) and is not usually associated with systemic symptoms.

Endogenous spread (hematogenous, lymphatic, or contiguous spread to skin):

• Scrofuloderma results from involvement of skin overlying a contiguous TB focus, usually a lymph node, bone, joint, or lacrimal gland. It is the most common form of CTB in children.
• Orificial TB is a rare mucosal autoinoculation TB from pulmonary, genitourinary, or intestinal disease shedding organisms that seed the adjacent orifices. 
• Lupus vulgaris results from hematogenous spread and is a progressive form of CTB seen in individuals with prior sensitivity and low immunity. 
• Tuberculous gumma (metastatic tuberculous abscess) is a rare form of hematogenous TB.
• Acute miliary TB is a rare but life-threatening form of hematogenous TB seen in patients with low immunity. 

Tuberculids: Tuberculids are presumed to be a hypersensitivity reaction to the TB organism and occur in patients with high immunity. There are 3 forms:

• Micropapular (lichen scrofulosorum) 
• Papular (papulonecrotic tuberculid)
• Nodular (erythema induratum). Erythema nodosum can be also seen in pulmonary TB. Lesions are typically asymptomatic but may be painful. Localized adenopathy may occur.

Tuberculosis due to BCG vaccination: Lupus vulgaris, scrofuloderma, erythema induratum, papulonecrotic tuberculids, and lichen scrofulosorum have all been described.

All forms of CTB including tuberculids require systemic treatment. Without treatment, lesions can persist for months to years. The prognosis of miliary TB is often poor.