yeast

hiiva An often humid, yellowish froth produced by fermenting malt worts, and used to brew beer, leaven bread, and also used in certain medicines.
A single-celled fungus of a wide variety of taxonomic families.
(quote-book)
A true yeast or (vern) in order Saccharomycetales.
(vern), Saccharomyces cerevisiae
A compressed cake or dried granules of this substance used for mixing with flour to make bread dough rise.
brewer's yeast, certain species of Saccharomyces, principally Saccharomyces cerevisiae and (taxlink).
Candida, a ubiquitous fungus that can cause various kinds of infections in humans.
The resulting infection, candidiasis.
puhekieltä A frothy foam.
1851, (w), (w):

But what most puzzled and confounded you was a long, limber, portentous, black mass of something hovering in the centre of the picture over three blue, dim, perpendicular lines floating in a nameless yeast.