There isn't anything magical about the disinfectant properties of alcohol, it is just a question of concentration. Anything above 20% ABV or so is going to be too toxic for microorganisms to survive in. Less than that, the doors are open for an infection. For those that might pan sanitizing a yeast packet before opening and pitching into beer, hey be my guest and risk loosing $40-60 worth of materials. The lower alcohol level of beer makes it very susceptible to infection.
As for beer in the Middle Ages being safer than ordinary water, that is true but not because of the alcohol present. It is because beer is boiled 45-90 minutes. The United States Centers for Disease Control and Prevention recommends vigorously boiling untreated water for 1 minute, 3 minutes if above 2,000m elevation. They also state that boiling is the most effective method for making water safe to drink. If a 1 minute boil is sufficient to sanitize drinking water, then 45 minutes is most certainly going to do the trick.
Originally Posted by wiltshiremead
I think there is lot more to this yeast, bacteria business than just sanitize them all.
If you are not convinced, I think you'd better wear a mask 24/7 because I know you have just breathed in that black mold spore
I can guarantee you that there is black mold spore floating in the air where you sit. Yes, it's everywhere but if you are healthy, it is all kept under check.
As Soyala said, somewhere between 75-90% of the North and South American indigenous population was wiped out by European diseases. And what about the Black Death plagues in the 1300's that killed off 1/3 of the European population? AIDS? While I certainly agree that there is more to microorganisms than simple destruction, some things will simply overwhelm the healthiest person.