I think discovery of treatments (drugs and vaccines) against many deadly diseases that were the major cause for high mortality rate in past, but it has been sorted out and result is now "overpopulation" which is impacting mankind in every possible way.
I personally consider that the "Genetic Code" (Blue print of life) is the most impactful discovery or invention as it is covering the basics of all investigations from medicines to vaccines, virus to transgenesis, from birth to death and so on.