Drug, substance that affects the function of living cells, used in medicine to diagnose, cure, prevent the occurrence of diseases and disorders, and prolong the life of patients with incurable conditions.

 Since 1900 the availability of new and more effective drugs such as antibiotics, which fight bacterial infections, and vaccines, which prevent diseases caused by bacteria and viruses, has increased the average American’s life span from about 60 years to about 75 years. Drugs have vastly improved the quality of life. Today, drugs have contributed to the eradication of once widespread and sometimes fatal diseases such as poliomyelitis and smallpox.