Everyone learns during their first brewing experience that you have to carefully sanitize anything that touches your wort after the boil to avoid infection.
What happens when wort gets infected? How do you identify an infection, and what can be done about it? Can you discover it before you bottle? Does infection mean losing the batch, or can you recover from it? If you can recover, how is the flavor of the beer affected? Is infection a strong possibility if you don't sanitize, or is just paranoid beer-lovers trying to avoid the worst-case scenario: wasting good beer?