Overview
Introduction
To understand the disease, start with the normal lung.
To understand the disease, start with the normal lung. Every breath carries air deep into the alveoli, the tiny grape like air sacs where oxygen crosses into the blood. Guarding those air sacs are alveolar macrophages, immune cells whose job is to swallow and destroy inhaled particles, including bacteria and fungal spores. In most inhaled exposures, these macrophages capture the invader, break it down, and the person is never the wiser. This is the defense that *Histoplasma* is unusually good at defeating. *Histoplasma capsulatum* is a dimorphic fungus, meaning it takes two shapes depending on temperature. In cool soil it grows as a mold that produces lightweight spores. When those spores are inhaled into the warm human lung, the fungus transforms into a yeast form. Here is the clever and dangerous part: instead of being destroyed by the alveolar macrophage that engulfs it, the yeast survives and multiplies inside that macrophage. The very cell meant to kill the fungus becomes its transport and shelter. Because macrophages travel through the lymphatic system and bloodstream, the organism can be carried from the...
