When I asked my teacher about why some physical descriptions in the bible or medical practices in the talmud seem impossible/wrong today he told me that the physical nature of the world changed, which he called meshane hateva. As a source he cited a mishna at the end of Sotah which talks about various curses to the physical world after the temple was destroyed. I'd like to know more about this idea. Are there any other, more explicit sources for it? Did things change all at once or gradually? Why do we still abide by certain health-related ideas from the talmud like not mixing fish and meat while not following others like turning a mill to relieve fever? Thank you.
There are rishonim and serious acharonim who discuss this idea outright. See Tosfos Moed Katan 11a ד"ה כוורא. See also the Kesef Mishna to hilchos de'os 4:18 who says that the nature of a certain cure may have behaved differently in one region than in another. See also the Yam Shel Shlomo Chulin perek 8 siman 12. See also the Chida in his peirush to Sefer Chasidim (peirush Azulai/Bris Olam) siman 477. See also the Chazon Ish Yoreh De'ah 5:3. See also Igros Moshe Even Ha'ezer chelek 2 siman 3 anaf 2 (the right column).
However, not everyone accepts this concept as an answer to questions of this variety. See Sefer Maharil Likutim siman 49* . See also Torah Chazal and Science by Rabbi Moshe Meiselman page 193 about the opinion of the Rashba. See also shu"t Chavas Yair siman 234 (little number 6 on sefaria) who brings down multiple opinions in the resolution of the question about the refuos in the gemara not working in modern times.
*I couldn't find this online, so I will quote the relevant part: אמר לנו מהר"י סג"ל כל הרפואות והלחשים שבכל התלמוד אסור לנסות אותם משום דאין אדם יכול לעמוד על עיקרם וכי לא יעלו בידם וילגלגו על דברי חכמים