The human body is home to trillions of bacteria, viruses and fungi cells. These cells can live on the skin or in the gut. Microorganisms provide a necessary function for human survival.
As the bacteria replicate they can undergo mutations. Sometimes, these mutations help them fight off antibiotics that would normally be effective. Antimicrobial use has contributed to the development of mutations causing resistance.
The resistant organisms proliferate overtime, becoming the predominant organism type. Sometimes, the resistant organisms pass along the instructions to other organisms, causing more widespread resistance.