When a beer tastes sour, phenolic or otherwise off, you frequently hear brewers describe it as being “infected.” Although we all understand what they are trying to say, a much better term to describe the beer’s condition is “contaminated.”
Infection occurs when a living organism is colonized by disease-causing organisms, such as bacteria, fungi or viruses. For example, a person suffering from a common cold is infected with a rhinovirus. Your body’s immune system is fighting off foreign invaders constantly and typically an intrusion of a foreign organism wouldn’t be termed an infection until the invader “establishes a beachhead” — survives inside you and begins reproducing. (When an organism is invaded by multi-cellular organism, such as a tapeworm or other parasite, the host is usually not described as being infected. Frequently, biologists say the host harbors the parasite.)
When beer has microorganisms in it that lead to sour, phenolic or any of the multitude of bad flavors and aromas rogue microorganisms produce, the beer is contaminated. Although beer is made with parts living organisms — carbohydrates from barley, acids and oils from hops — and fermented by yeast (a living organism), beer is not itself a living organism. The microorganisms are not infecting the beer; neither are they causing the beer to be diseased.
When non-living things harbor bacteria — for example, water — we would say that the water was contaminated by bacteria, not infected with bacteria. Some dictionaries allow for the word “infect” to be used synonymously with “contaminate,” but “contaminate” is a word that describes the condition exactly, and should be preferred.
Whenever someone hands me a beer and says, “Is this infected?” I say, “I know it’s not infected, by definition, but let me see if it tastes contaminated.”