Antimicrobial resistance (AMR), including antibiotic resistance, is the resistance of a microbe to an antimicrobial medication that used to be effective in treating or preventing an infection caused by that microbe. There are three main ways by which resistance can occur: by natural resistance in certain types of bacteria, by genetic mutation, or by one species acquiring resistance from another. Resistance can happen spontaneously owing to random mutations, to a buildup of resistance over time, or to misuse of antibiotics or antimicrobials, although the latter two pathways are the most important. Resistant microbes become increasingly difficult to treat, requiring alternative medications or higher doses, both of which may be more costly or more toxic. Microbes which are resistant to multiple antimicrobials are called multidrug resistant (MDR); in the press, these organisms are often referred to as superbugs. Antimicrobial resistance is an increasingly problematic issue that leads to millions of deaths every year. A few infections become completely untreatable due to resistance. All classes of microbes develop resistance (fungi, antifungal resistance; viruses, antiviral resistance; protozoa, antiprotozoal resistance; bacteria, antibiotic resistance). Of those categories, bacterial antibiotic resistance poses the largest threat to public health.