Alcohol as disinfectant

A German physician, Dr Dietrich Weller of Leonberg wrote this to me recently:

Dear Mrs. Robinson,

Thank you for many hours of reading your books and tasting wine accompanied by your suggestions. I have just finished reading "Der Degustationskurs" (Wine-tasting Workbook/How to Taste[link to books] in German), which again gave me quite a lot of good ideas and affirmations. But I stumbled over a small but important mistake, and therefore I would like to write to you.

In the chapter "Offizielle Weinproben" you write on page 83 (German edition)that we could easily share the same glass because alcohol is a...