Answer:
Spherules are the form that the fungus takes in tissue. In nature, the fungus grows in soil and appears in the mycelial form similar to bread mold. Portions of the hair-like mycelia break off into arthroconidia ("spores") and become airborne when the soil is disturbed. The arthroconidia are extremely small and light and may be carried many miles by the wind. Fifteen trillion of the arthroconidia would fit into a cubic inch. There has been documentation of a severe dust storm occurring in Bakersfield, California, an endemic area, blowing as far north as San Francisco where it is not normally found, and causing an epidemic in the Bay area.