Patients become septic, or develop sepsis to be more accurate, due to bacterial, fungal or parasitic infection(malaria) within the blood stream. For obvious reasons our blood is a perfect culture media and supports bacterial growth easily.This can come from an external source (cut, scratch, infected needle) that contains bacteria. It can also develop due to UTI and pneumonia that has either been 1.) untreated, 2) improperly treated (wrong abx, eg). or 3.) undertreated (too short a course, nonadherence to rx). Immune compromise is not necessary, but comes along with the very young and the very old, as well as obvious reasons (HIVetc.) One main cause which is often not noted is from invasive lines. I had a gentleman on Psych once, who had blown his renals due to Lithium tox , had central dialysis catheter, and his main sx was HYPOTENSION! On further study, had elevated wbc. We sent to the unit, with hypotensive shock due to line sepsis.
Failure to diagnose can also result in horrendous situations. One guy came in to ER with what was diagnosed as a "sprain". He had swelling and erythema in the ankle. Was sent home on Lortab or some such. Came back in 2 weeks later with leg swollen to the knee, had to have bilateral fasciotomy, vac, and skin graft. Nearly lost his leg!!