Facts from "Discover" magazine

96. And Here's Why You Have an Appendix:
When you're sick, it re-boots your gut with good bacteria.
by Josie Glausiusz

In September, a team of surgeons and immunologists at Duke University proposed a reason for the appendix, declaring it a “safe house” for beneficial bacteria. Attached like a little wiggly worm at the beginning of the large intestine, the 2- to 4-inch-long blind-ended tube seems to have no effect on digestion, so biologists have long been stumped about its purpose. That is, until biochemist and immunologist William Parker became interested in biofilms, closely bound communities of bacteria. In the gut, biofilms aid digestion, make vital nutrients, and crowd out harmful invaders. Upon investigation, Parker and his colleagues found that in humans, the greatest concentration of biofilms was in the appendix; in rats and baboons, biofilms are concentrated in the cecum, a pouch that sits at the same location.
The shape of the appendix is perfectly suited as a sanctuary for bacteria: Its narrow opening prevents an influx of the intestinal contents, and it’s situated inaccessibly outside the main flow of the fecal stream. Parker suspects that it acts as a reservoir of healthy, protective bacteria that can replenish the intestine after a bacteria-depleting diarrheal illness like cholera. Where such diseases are rampant, Parker says, “if you don’t have something like the appendix to harbor safe bacteria, you have less of a survival advantage.”