Tuberculosis (TB) is a contagious infection primarily affecting the lung parenchyma but which can also affect other body parts. TB can be classified based on disease development, presentation, and the affected anatomical site.

The first classification is based on the development of the disease, and it includes the following categories:

• Class zero: The individual has no known TB exposure and has a negative tuberculin skin test.

• Class one: The individual has been exposed to TB but shows no evidence of infection, as their tuberculin skin test is negative.

• Class two: The individual has a positive tuberculin skin test but no clinical, bacteriological, or radiographic evidence of TB disease.

• Class three: the individual has clinically active TB with a positive tuberculin skin test and also has clinical, bacteriological, or radiographic evidence of current TB disease.

• Class four: the individual does not have clinically active TB. In these cases, the patient has a history of previous episodes of TB or has abnormal but stable radiographic findings in the absence of current symptoms. They may have a positive tuberculin skin test.

• Class five: TB is suspected, and diagnosis is pending.

Tuberculosis can be further classified based on its presentation, including the following categorizations:

• Primary Tuberculosis occurs when a person is infected with the TB bacteria for the first time. The immune system usually manages to contain the bacteria, and the person may not experience any symptoms.

• Post-primary or Secondary Tuberculosis occurs when an individual, previously sensitized to Mycobacterium tuberculosis, gets re-infected or when the dormant bacteria become active. This form of TB is often symptomatic and can be more severe.

• Latent TB Infection, or LTBI, is a condition in individuals infected by Mycobacterium tuberculosis. The bacteria are dormant and do not cause any symptoms. People with LTBI do not feel sick and cannot spread the infection to others.

Tuberculosis can also be classified based on the anatomical site of the disease. There are several types:

• Pulmonary tuberculosis is the most common type of TB, affecting the lungs. Symptoms often include a persistent cough, chest pain, hemoptysis, fatigue, weight loss, reduced appetite, chills, fever, and night sweats.

• Extrapulmonary Tuberculosis refers to TB that occurs outside of the lungs. The bacteria can affect various parts of the body.

Some common forms of extrapulmonary TB and their symptoms include:

• Lymphatic TB affects the lymph nodes, usually in the neck,

• Skeletal TB affects the spine and other bones, resulting in bone pain and possible deformities,

• TB Meningitis affects the meninges and can provoke headaches, neck stiffness, and neurological issues,

• Genitourinary TB affects the kidneys, bladder, and reproductive organs, resulting in flank pain or bloody urine,

• Gastrointestinal TB affects the stomach, intestines, and other parts of the digestive system, causing abdominal pain or bloody stool.

• Disseminated TB refers to a broader condition in which mycobacterium tuberculosis has spread from the initial site of infection (usually the lungs) to other organs via the bloodstream, lymphatic system, or direct extension. These organs can include the liver, spleen, kidneys, bones, and brain.

• Miliary TB is a specific type of disseminated TB that occurs when mycobacterium tuberculosis bacteria disseminate widely through the bloodstream and form small, millet seed-sized lesions (1–5 mm) in multiple organs. These lesions are characteristic of miliary TB and can be seen in organs like the lungs, liver, spleen, and bone marrow. The term "miliary" derives from the resemblance of the lesions to millet seeds observed on chest radiographs or during histological examination.