Hemolytic-uremic syndrome (or haemolytic-uraemic syndrome), abbreviated HUS, is a disease characterized by hemolytic anemia (anemia caused by destruction of red blood cells), acute kidney failure (uremia), and a low platelet count (thrombocytopenia). It predominantly, but not exclusively, affects children. Most cases are preceded by an episode of infectious, sometimes bloody, diarrhea acquired as a foodborne illness or from a contaminated water supply and caused by E. coli O157:H7, although Shigella, Campylobacter and a variety of viruses have also been implicated. It is now the most common cause of acquired acute renal failure in childhood. It is a medical emergency and carries a 5–10% mortality; of the remainder, the majority recover without major consequences but a small proportion develop chronic kidney disease and become reliant on renal replacement therapy.The primary target appears to be the vascular endothelial cell. This may explain the pathogenesis of HUS, in which a characteristic renal lesion is capillary microangiopathy.HUS was first defined as a syndrome in 1955. The more common form of the disease, Shiga-like toxin-producing E. coli HUS (STEC-HUS), is triggered by the infectious agent E. coli O157:H7. Certain Shiga toxin secreting strains of Shigella dysenteriae can also cause HUS. Approximately 5% of cases are classified as pneumococcal HUS, which results from infection by Streptococcus pneumoniae, the agent that causes traditional lobar pneumonia. There is also a rare, chronic, and severe form known as atypical hemolytic uremic syndrome (aHUS), which is caused by genetic defects resulting in chronic, uncontrolled complement activation. Both STEC-HUS and aHUS cause endothelial damage, leukocyte activation, platelet activation, and widespread inflammation and multiple thromboses in the small blood vessels, a condition known as systemic thrombotic microangiopathy (TMA), which leads to thrombotic events as well as organ damage/failure and death.