Fungi are microscopic living vegetable organisms that have no chlorophyll (the pigment that makes plants green). Fungi grow in branch filaments called Mycelium and reproduce through spores.
When we see moldy bread, we are seeing the Mycelia of a fungi (generally Penicillium). However, the allergic symptoms are caused mainly by the spores (the equivalent of the seeds).
Spores are very small and light, and therefore can be carried by the wind long distances. Fungi can grow in houses or outdoors. They usually develop in humid and warm environments and prosper in natural areas with organic matter in decomposition.