Histoplasmosis:

      Every year, hundreds of thousands of people worldwide contract the lung disease histoplasmosis. It's transmitted through airborne spores that you breathe into your lungs when you work in or around soil that contains a fungus called Histoplasma capsulatum. Farmers, landscapers, construction workers and people who have contact with birds or bats are especially at risk.

Most people with histoplasmosis never develop signs and symptoms and aren't aware they have the disease. But for some people — primarily infants and those with compromised immune systems — histoplasmosis can be much more serious.

Effective treatments are available for even the most severe forms of histoplasmosis. But these therapies often involve extensive hospital stays and can cause serious side effects, so people with compromised immune systems should do everything possible to avoid histoplasmosis infection.

Histoplasma capsulatum is primarily found in the temperate regions of the world and is the most common fungus in the United States. It's endemic in the Ohio, Missouri and Mississippi river valleys, where the great majority of people have been exposed.

The fungus thrives in damp soil that's rich in organic material, especially the droppings from birds and bats. For that reason, it's particularly common in chicken and pigeon coops, old barns, caves and parks.

Birds themselves aren't infected with histoplasmosis — their body temperature is too high — but they can carry H. capsulatum on their wings, and their droppings support the growth of the fungus. Birds commonly kept as pets, such as canaries and parakeets, aren't affected. And although bats, which have a lower body temperature, can be infected, you can't contract histoplasmosis from a bat or from another person.

Instead, you develop histoplasmosis when you inhale the reproductive cells (spores) of the fungus. The spores are extremely light and float into the air when dirt or other contaminated material is disturbed. That's why a high number of cases occur in farmers, landscapers, construction workers, people living near construction sites and spelunkers. In one instance, an entire Boy Scout troop became infected after cleaning up an old park filled with roosting starlings. In another, 14 healthy young college students developed histoplasmosis when they were exposed to bat guano in a cave in Nicaragua.

                                                              
Histoplasmosis and your lungs:
Your lungs are two spongy organs, each surrounded by a moist membrane (the pleura). When you inhale, air is carried through the windpipe (trachea) to your lungs in two major airways called bronchi. Inside your lungs, the bronchi subdivide into a million smaller airways (bronchioles), which finally end in clusters of tiny air sacs called alveoli.

Because the spores of H. capsulatum are no more than 2-millionths of a meter in diameter, they can easily enter your lungs and settle in the small air sacs. There, the spores are trapped by macrophages — immune system cells that attack foreign organisms. The macrophages carry the spores to lymph nodes in your chest, where they continue to multiply. This may lead to inflammation, scarring and calcium deposits. In cases of heavy infection, the lymph nodes may become so enlarged that they obstruct your esophagus or your lungs' airways.

Most often, however, you're not likely to have noticeable signs and symptoms, and the infection clears on its own without treatment. But if your immune system isn't able to eliminate the spores, they can enter your bloodstream and travel to other parts of your body. In that case, you may develop a variety of severe problems that can be fatal if not diagnosed and treated quickly.

Fibrosing mediastinitis:

      A rare but severe late complication of histoplasmosis, fibrosing mediastinitis occurs when scar tissue from lymph nodes in the chest invades and blocks adjoining structures, especially the esophagus and large blood vessels. Signs and symptoms, such as a cough that brings up blood, chest pain and breathlessness, usually don't appear until the disease is quite advanced. When structures in both lungs are affected, fibrosing mediastinitis can be life-threatening.