Overuse of antibiotics has led to microbes that are increasingly resistant to treatment. When a species of bacteria is exposed repeatedly to a medication, a small segment of this bacterial population can develops an advantageous mutation. This mutation allows these particular bacteria to overcome the antibiotic. This new strain eventually “outcompetes” its relatives, spreading widely to cause infections.
This process has occurred more and more in recent decades. Misuse and overuse of antibiotics, combined with a lack of new medications have played a major role in this medical crisis. Treatment resistant infections take the lives of nearly 5 million people every year. By 2050, experts predict that 10 million deaths could be caused by treatment resistant microbes, exceeding the number of people killed by cancer every year.
Check out this excellent video from the BBC on antibiotic resistant bacteria.