Pasteur first formulated the idea that bacteria are responsible for the diseases of fermented liquids; the corollary of this was a demand for pure yeast.
About this time Hansen, who had long been engaged in researches on the biology of the fungi of fermentation, demonstrated that yeast free from bacteria could nevertheless occasion diseases in beer.
Having found that some of the commonest diseases of beer, such as yeast turbidity and the objectionable changes in flavour, were caused not by bacteria but by certain species of yeast, and, further, that different species of good brewery yeast would produce beers of different character, Hansen argued that the pitching yeast should consist only of a single species - namely, that best suited to the brewery in question.
The diseases or sicknesses of beer and wine had from time immemorial baffled all attempts at cure.
Specifically, a virus or bug passed to a pig is considered a huge threat in the medical community, because pigs can pass their diseases onto humans.