Mold

Mold is an everyday name, not a formal group, for fungi that grow as many-celled filaments rather than as single yeast cells or as large mushrooms. Their threads, the hyphae, branch to form a mycelium, and the visible fuzz or colored patches are usually masses of spore-bearing hyphae. Molds come from several unrelated fungal lineages; familiar ones include Penicillium, Aspergillus, Cladosporium, Mucor and Rhizopus, the black bread mold. Some fungi, called dimorphic, switch between a mold form and a yeast form depending on temperature.

Molds spread mainly by releasing huge numbers of tiny asexual spores that travel on air, so their spores are almost everywhere, indoors and out. They need moisture to grow, which is why mold appears after leaks or floods on wood, paper, drywall and fabric, and on bread, fruit and stored grain. As decomposers they recycle dead material. Some make valuable products, such as penicillin from Penicillium and the enzymes that ferment soy sauce and sake; others produce mycotoxins, such as the aflatoxins of Aspergillus flavus, potent causes of liver cancer.

Damp, moldy buildings are linked to coughing, wheezing and worse asthma, and in people with weakened immune systems certain molds cause serious infections such as invasive aspergillosis and mucormycosis.