Since the 1940s, when penicillin began to be used, antimicrobial drugs, such as antibiotics, have been successfully used to treat patients with bacterial and other infectious diseases. However, over time, many infectious microorganisms have adapted to the drugs designed to kill them, making the products less effective and hampering the treatment of patients. Most groups of microorganisms can develop resistance, including bacteria, fungi, and protozoa. As an unwanted advancement on the phenomenon, a growing number of disease-causing microorganisms are resistant to one or more antimicrobial drugs used for treatment (what are termed multidrug-resistant organisms [MDROs]).