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While other comments have clearly shown that the alcohol in beer isn't strong enough to disinfect it, the way beer is made is that you first have to disinfect the water you're going to use (by boiling it) and then you store the beer in conditions that stop pathogens getting in. So for those reasons the beer would be safer than water, but you'd have just as much success just boiling some water and using it straight away. The other advantage of beer over water is that if it does get pathogens in it, then it is really obvious, and people know not to drink foul beer [citation needed].