In fact, according to the Centers for Disease Control and Prevention (CDC), all bacterial infections in the world are slowly becoming resistant to antibiotic treatments. That’s because disease-causing bacteria are living organisms that constantly evolve, enabling them to adapt to new environments. Antibiotic resistance develops over time — it can start from even a very small number of microbes within a population that have genes that allow them to continue to grow, despite the use of drugs that would normally kill them.

Researchers suggest that some microbes are able to survive antibiotic treatments because they swap genes with each other, making them drug-resistant. In any case, the bacteria that survive an antibiotic treatment eventually outnumber the population of bacteria that are susceptible to the drug.

Here are 6 superbugs that can be challenging to treat.