Brettanomyces is the by-product of yeast activity in a wine, though not the good yeast that converts sugar to alcohol. It is usually present in oak barrels, though it does infect entire cellars, and is mostly closely associated with the wines of the Southern Rhône and to a certain extent, Tuscany -- though it can, and does, appear in every major wine-producing region.
In low concentration Brett can add great complexity to a wine’s bouquet. The typical positive descriptors tend to be a little funky, like barnyard, horse blanket, horsey, sweaty and/or cheesy. In higher concentrations, the aromas of Brett are typically created by three distinct chemical compounds, one of those -- 4-ethylphenol -- smells exactly like Band-Aids!