Infections are caused by bacteria that invade various tissues of the body or grow in the bloodstream. For about the past 80 years, doctors have used antibiotics to kill bacteria and cure infections. But millions (and sometimes billions) of bacteria are often present at any one time, and individual bacterial organisms may vary slightly with regard to their genetic makeup. These genetic differences allow some bacteria to produce chemicals that destroy the antibiotic that is being used to treat the infection. Overuse of antibiotics has been linked to the emergence of antibiotic resistance when the bacteria are able to adapt to the presence of these medicines by altering their genetic makeup. The resistant bacteria can eventually multiply and become more common in the environment. Researchers have responded to this threat by developing new antibiotics that are able to avoid being destroyed by the bacterial chemicals. Recently, a new and dangerous threat to successful antibiotic treatment has been identified: Researchers have found that bacteria can exchange among themselves packets of genetic material that make them resistant to antibiotics. Using this mechanism of exchange, some bacteria that are not usually resistant to antibiotics, such as Klebsiella pneumoniae, have developed strains that are resistant to several antibiotics at the same time. These strains are known as extended-spectrum β-lactamase (ESBL)–producing K. pneumoniae.