Infectious diseases are the second leading cause of mortality, causing about 20% of global deaths according to the World Health Report 2003. Among these, respiratory and other bacterial infections account for four million deaths each year while tuberculosis is responsible for another 1.5 million fatalities.
Antibiotics have saved millions of lives since their introduction in the 1940s, and have contributed much to the increase in life expectancy around the world in the past century. Yet drug-resistant pathogens have been on the rise in recent decades. Resistance may be due to exclusion (stopping the drug from entering the bacteria), efflux (pumping the drug out of the bacteria), inactivation of the drug (breakdown or chemical modification), or modification of the cellular target (e.g., point mutations within the active site). Increasingly, infections that were once treatable with antibiotics are becoming difficult or impossible to treat.