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Histoplasmosis is caused by a dimorphic (two forms) fungus named Histoplasma capsulatum. The dimorphic fungus has a branching (mycelial) phase consisting of branches and spores that can be inhaled while they are airborne and may reach the lung alveoli. Macrophages (immune system cells that protect the body by engulfing foreign invaders) surround and engulf H. capsulatum. The fungus then changes inside the macrophages to the yeast form in about 15-18 hours. In most cases, the macrophage response kills the yeast. When macrophages fail to kill all the yeast, a variation of the disease develops because the yeast form multiplies and invades other cells. The larger the number of mycelia and spores the person is exposed to, the more likely the person will develop symptomatic disease. Severe histoplasmosis occurs when the yeast forms are spread by the blood and lymphatic systems to other organs. Histoplasmosis is sometimes referred to according to the severity of the disease:
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