Secondary tuberculosis infection, a major type of tuberculosis, is generally caused by the onset of tuberculosis through latent lesions left in the lungs by the primary infection that rekindle when the body is immunocompromised, or by reinfection with Mycobacterium tuberculosis. It is mostly seen in adults, where endogenous onset is generally the main route. Secondary TB has a variety of pathological changes and can manifest in various forms such as infiltrative TB, hyperplastic TB, cavitary TB, and TB spheres, which are prone to caseous necrosis and cavity formation. The general excretion of bacteria is high and is of epidemiological importance. Generally TB is transmitted through air droplets. When a patient coughs, sneezes, or speaks loudly, the exhaled gas containing Mycobacterium tuberculosis is dispersed into the air and then forms droplet nuclei, which are suspended in the air and can also be attached to dust in sputum. When a healthy person inhales droplets or dust of Mycobacterium tuberculosis, it can cause infection.