That is because gene expression is a process in the synthesis of proteins and without proteins our organisms could not function properly. Almost all things that occur in our body are results of proteins being synthesized and these proteins travel through our body activating other processes. When not synthesizing proteins, different types of RNA are synthesized.
Antibiotics killed the bacteria that cause infection but they can also kill beneficial bacteria . thus Ms ABC developed a yeast infection and diarrhea after course of antibiotics because antibiotics distrub natural balance of beneficial bacteria in her intestine.