Histoplasmosis is an infection of the lungs. This infection is caused by a germ called a fungus. The fungus grows as a mold in the dirt. Bird and bat droppings make the dirt a good place for the germs to grow. The infection spreads by breathing the germ spores in the air. The spores come from dirt being moved around. You cannot catch this illness from some other person or animal. Some cases are mild and go away without treatment. Other cases will need treatment.