The diuretic effects of inorganic mercury salts, which were used to treat syphilis, had already been noted in the 16th century by Paracelsus; in the 19th century they were used by noted physicians like John Blackall and William Stokes. In the meantime, cannulae (tubes) invented by English physician Reginald Southey in 1877 was another method of removing excess fluid by directly inserting into swollen limbs. Use of organic mercury compounds as diuretics, beyond their role in syphilis treatment, started in 1920, though it was limited by their parenteral route of administration and their side-effects. Oral mercurial diuretics were introduced in the 1950s; so were thiazide diuretics, which caused less toxicity, and are still used today. Around the same time, invention of echocardiography by Inge Edler and Hellmuth Hertz in 1954 marked a new era in the evaluation of heart failure. In the 1960s, loop diuretics were added to available treatments of fluid retention, while a patient with heart failure received the first heart transplant by Christiaan Barnard. Over the following decades, new drug classes found their place in heart failure therapeutics, including vasodilators like hydralazine; renin-angiotensin system inhibitors; and beta-blockers.
Principal ideal domain, an algebraic structure
Process identifier, a number used by many operating systems to identify a process

Organisations
Police Intelligence Department, a staff department of the Singapore Police Force
Political Intelligence Department (disambiguation), multiple organisations
Politieke Inlichtingen Dienst (Political Intelligence Department), was the main security agency for the Dutch East Indies
Prague Integrated Transport (Pražská integrovaná doprava – PID in Czech), an integrated public transport system in Prague

Other uses
Pathways into Darkness, a 1993 video game by Bungie
"Paul is dead", a 1960s urban legend that Paul McCartney was dead
Pid (video game), a 2012 video game by Might and Delight
Project Initiation Documentation, in project management
Party identification, the political party with which an individual identifies
Everett Pid Purdy (1904–1951), American athlete who played in both Major League Baseball and the National Football League

See also
PIDS (disambiguation)