There are many types of germs, or pathogens, that can enter the human body. These include viruses, fungi, bacteria, and protozoa. Once inside the body, the primary goal of a germ is to survive and reproduce.
Pharmaceutical drugs are designed to target these germs and kill them or prevent them from reproducing inside the body. If a germ continues to reproduce during treatment, it can alter itself -- or mutate -- to avoid the drugs. This is called drug resistance.
When drug resistance occurs, the drug -- or combination of drugs -- loses its ability to block the germ from reproducing. Over time, the treatment can stop working completely. It is important to prevent germs from reproducing during treatment to prevent drug resistance from occurring.