system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 how do people with hemmoroids not constantly get e.coli infections?
gpt
Physician here. Hemorrhoids are veins that have becomes swollen In the rectum. Occasionally they may bleed which leaves an open communication between the blood stream and the colon but they don't often get infected. Why? Think of it as a small weak fish (bacteria) trying to swim upstream against a fast current.  It doesn't get anywhere right? That's the pressure behind the vein. Now imagine the same stream with a bunch predator fish swimming around eating the small fish.  This is the immune system in and around the rectum.  So we have a two protection systems against infection.  

The most common reason hemorrhoids become infected is when the blood supply to the hemorrhoid ceases (strangulated or thrombosed). So now there is a dam, river comes to a standstill and the predator fish can't get in so the little fish (bacteria) can go wherever he likes including right into the now dead tissue of the hemorrhoid and wreak havoc.