A batch of beer is said to have an infection when a bacteria or harmful yeast has entered it and created off flavours in the beer.
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 Avoiding infections
The best way to avoid infections is to follow good cleaning and sanitation procedures. Key to this is ensuring that all equipment that comes into contact with wort is clean (free of any physical debris- sanitation is impossible if there is dirt on equipment) and sanitised. In a home brew situation, sterilisation is virtually impossible due to how beer is made in a home situation. However, it is always best to aim for as much sanitation as possible to reduce any risk.