Caveat: The below discussion contains opinion! So, here is the thing: we homebrewers practice sanitation vs. sterilization so technically every beer we brew contains unwanted microorganisms. A homebrewed beer is typically considered to be infected if there are unwanted and undesirable qualities to the resulting beers. As one example, if the beer has off-flavors due to the ‘excess’ presence of unwanted microorganisms this beer would be described as being infected. There are other examples which could be mentioned. One of the strategies that can be employed to mitigate the case of an infected beer is to pitch a lot of yeast so they ‘out muscle’ the unwanted microorganisms. So, what ‘level’ of sanitation is needed in order to produce quality homebrew? There is no single answer here since there are so many variables. If during the pre-fermentation aspect the ‘level’ of sanitation is ‘low’ a homebrewer who pitches a ‘boat load’ of yeast can still produce a beer with no perceptible off-flavors. Each homebrewer needs to recognize what ‘level’ of sanitation is needed in their homebrewery taking into consideration the holistic aspects of their brewing process. Cheers!