Yesterday morning in shul I saw a new sefer that was just printed on hilchos גילוי. You might ask what is hilchos גילוי?
The Gemara in Avoda Zara (and other places) says that you are not allowed to drink from water (and other liquids) left uncovered because we are afraid that a snake may have injected poison into the liquid without your knowledge. This is know as גילוי.
The Rif quotes להלכה. However, the Rishonim starting with R' Tam say that this no longer applies where we live and in fact in יו"ד סימן קט"ז the Shulchan Aruch paskens that nowadays there is no issur of גילוי.
The question can be asked why is this not noheg nowadays? After all דבר שבמנין צריך מנין אחר להתירו. The Gra there on Shulchan Aruch comments that the gezera was only made in places where there was a problem of גילוי, in our time and location there is no problem and therefore the gezera does not apply. Others say that there never was a gezera. Chazal were giving us advice vis a vis a danger but it was never codified as a gezera.
Based on all this, why nowadays is a sefer on גילוי coming out? After all it would seem that the din does not apply now. The answer is that the Gra himself was makpid on גילוי. He held that גילוי was a regular gezera and applies in any time and any place (this is the Gra לשיטתו by many other similar things). The Chazon Ish, Steipler, R' Chaim Kanievsky and others are makpid on גילוי today.
The sefer has the following chakira. When Tosafos said that there was no גילוי in their location what did they mean?
1. They meant Europe as opposed to the Middle East, in Europe there are fewer snakes
2. They meant that in their time there were fewer snakes in the world
3. They meant that their living conditions were such that there was no problem with snakes.
The author rejects possibilities 2 and 3 and goes with the first explanation. Based on that he is bothered how can we (who live in Israel) be סומך on the heter of Tosafos, here there are more snakes?
The answer IMHO is very simple. Whether or not Tosafos said it there is no question that today our living conditions are much different and improved over Chazal and we don't have snakes in our houses, certainly not in our refrigerators. Therefore, the gezera does not apply because there is no problem of גילוי given our modern living conditions.