Hemorrhoids (alternative spelling Haemorrhoids) are the most common anorectal disorder, and are frequently seen in primary care clinics, emergency wards, gastroenterology units, and surgical clinics. More than half of all people will at some point develop symptomatic hemorrhoids.
"The exact cause of hemorrhoids is unknown. Several contributing factors have been implicated, including the upright posture of humans, aging, pregnancy, heredity, constipation or chronic diarrhea, and spending excessive periods of time on the toilet (ie, reading, straining)."
Source : Perm J. 2007 Spring; 11(2): 74–76.
It should be noted that all healthy individuals naturally have hemorrhoids to some degree - they are believed to act as an anorectal cushioning mechanism.