For more than 100 years, the general assumption has been that 98.6 degrees is the normal average body temperature. But studies show it has decreased in recent years.
https://www.king5.com/mobile/article/news/verify/health-verify/average-normal-body-temperature-not-98-6-degrees/536-f7d4b9e0-a18c-452f-b62a-247fffa22051