It's Saturday night and more or less time to mark the end of Shabbas, and you're walking outside the city and you see a flame. If the majority of the city's inhabitants are gentiles, it is a fairly good possibility that the flame was used in connection with work (or worse, in an idolatrous service), so don't even think about saying a blessing over it. On the other hand, if it is mostly a Jewish town, go right ahead and recite the blessing. (If you are now observing Pesach's dietary restrictions and you live in a "gentile town," you have a pretty good idea why these considerations would be part of our text.)
If you're in the study hall (it's still Satuday night) and someone brings in a flame, Shammai would have you say the blessing quietly to yourself so as not to interrupt the study of others, but Hillel puts the communal experience above the need of the individual to study and encourages interruption of study for a communal blessing. This appears to be one of the rare instances where Shammai prevails, and he is supported by Rabban Gamliel (remember him?):
ArtScroll footnotes this passage with a short history of the sneeze, beginning with the explanation that saying "Health!" is:
The members of Rabban Gamliel's household would not say "Health!" in a study hall because this would cause a disruption of Torah learning in the study hall.(53a).
As is customary when someone sneezes (Rashi; see Magen Avraham 230:6). [The source for this custom is stated in Pirkei DeRabbi Eliezer 52 (cited by Gilyon HaShas): From the time of creation until Jacob's time, no man would take ill prior to his death; indeed, illness as such did not exist at all, and there was no warning of a person's imminent demise. Rather, a man walking on the road or in the marketplace would suddenly sneeze, and his soul would exit via his nostrils. Hence, a sneeze was the precursor of death. Jacob, however, beseeched Hashem for mercy, praying that his soul not depart suddenly from this world, so that he would have time to instruct his sons before his passing. Hashem granted his request, and from then on, people would take ill prior to their death. Therefore, when one sneezes, it is customary for others to respond with "Health!" (the equivalent of our "Gesundheit" or "God bless you"), in recognition of the fact that the sneeze is no longer a sign of impending death.]This is quite timely now, at the height of hayfever season when others pray for relief from month-long sneezing fits and are given Claritin. It also brings to mind a story from the other Jewish tradition (the Borscht Belt), where a fellow reported to his friend that he had a condition whereby with every sneeze he felt as if he was having a powerful orgasm. His friend asked him what he took for this condition. "Pepper," he replied.