Ergot

[David Morris]

http://www.boingboing.net/2011/01/19/toxic-fungus-infiltr.html

While we're well aware of the danger inherent in these little black
pods today (and can remove them from infested grain), Medieval
Europeans were not so fortunate. Ergots were routinely milled into
flour, and in bad years could reach frighteningly high levels. The
ergots were not identified as a source of this disease until 1670
(partially because these little sclerotia were so common that they
were included in botanical illustrations of rye!)