Here's why:

* Microbiology: This branch of biology focuses on the study of microorganisms, which includes bacteria, viruses, fungi, and protozoa. Understanding how these organisms live, reproduce, and interact with their environment is crucial to developing methods for killing them.

* Infectious Disease: This field deals with the study, prevention, and treatment of diseases caused by microorganisms. Killing germs is a fundamental part of preventing and treating infectious diseases.

While there are other branches of science that might be involved in the process of killing germs (like chemistry for developing disinfectants), microbiology and infectious disease are the primary fields that focus on the underlying principles and practical applications of killing germs.