Some environmental bacteria can grow in ventilation systems, but these are raely a threat to healthy people. They can, however, be a nosocomial problem. The photomicrograph at the right shows lung tissue infected with a growing mycelium of aspergillus. Fungi differ significantly, in certain respects, from most other airborne pathogens, such as bacteria, viruses, and protozoa. Fungi can exist outdoors and enter the building through the air intakes.
Fungi are normally harmless and non-parasitic.
Fungal infections inevitably result from fungi being in the wrong place, often as the result of poor cleanliness or improper design of ventilation system components. The true fungi are the Eumycetes. Some of the fungi, the mushrooms, yeasts and some of the molds are extremely beneficial to us. They assist the production of cheese, antibiotics, yogurt, wine and beer. Some fungi, like the blights, can cause extensive crop damage.