The virulence of Mycobacterium tuberculosis mainly depends on the continuous growth and reproduction in the body. There are no cell appendages such as endotoxin, exotoxin, flagella and spores. Under normal circumstances, a small amount of Mycobacterium tuberculosis enters the human body and is swallowed by the body's macrophages, digested by some enzymes in the cells and then dies. However, in many cases, a large number of lipid components in the cell wall are difficult to be digested by enzymes, and Mycobacterium tuberculosis in macrophages precipitates and becomes a so-called "intracellular parasite", which continues to reproduce slowly. After bacteria multiply to a certain number, macrophages swell and collapse, bacteria disperse, and then they are swallowed again and again, again and again. At this point, the human body has no symptoms and no tuberculosis. People in this situation are healthy carriers. If the body's immunity declines for some reason, macrophages are destroyed, and Mycobacterium tuberculosis proliferates in large numbers, spreads into the blood or directly invades parasitic organs (mainly lungs), patients will have tuberculosis. If there is a problem with the body's nutrition, the patient will have cachexia and even death.