Some can grow at subfreezing temperatures, but most become dormant. Snow cover lowers the outdoor mold count drastically but does not kill molds. After the spring thaw, molds thrive on the vegetation that has been killed by the winter cold. In addition, molds growing indoors can cause perennial allergic rhinitis even in the coldest climates. There are thousands of types of molds and yeast, the two groups of plants in the fungus family.
Yeasts are single cells that divide to form clusters.
Molds consist of many cells that grow as branching threads called hyphae. Although both groups can probably cause allergic reactions, only a small number of molds are widely-recognized offenders. The seeds or reproductive particles of fungi are called spores. They differ in size, shape, and color among species. Each spore that germinates can give rise to new mold growth, which can produce millions of spores.