Bacteria within the bloodstream of an animal is called bacteremia or septicemia. Newborn foals are uniquely susceptible to this problem. Unlike healthy adult horses, whose blood defenses usually immediately destroy bacteria in the bloodstream, foal immune defenses are weaker, allowing bacteria to survive and spread.

Septicemia usually results from intestinal bacterial infection, which then spreads into the bloodstream. Foals that have not gained antibody from timely ingestion of the mare's first milk (colostrum) are highly susceptible.
Less frequently, foals became ill in the uterus prior to birth and are born with bacteria within their bloodstream.

Septicemia is unfortunately quite common. It is a life threatening emergency that will require hospitalization, intensive care and monitoring, iv-fluid therapy, antibiotics and other medications. A sick baby foal (a few days old) with diarrhea is septicemic until proven otherwise and requires rapid veterinary involvement. Bacteria within the bloodstream can also spread to other organs, and especially to joints and tendon sheaths, causing infection of those structures. Foals with an infected joint usually develop a sudden onset of joint swelling and lameness.