I’m not even going to bother with an introduction on this one. That pink stuff is bacteria. If your toilet bowl (or sink, or whatever) has stains like the photo below, it’s ready for a cleaning. Now.
More specifically, the bacteria in question is Serratia marcescens, a species which thrives in damp environments (like bathrooms) and feeds on phosphates (like those contained in shampoo and soap). It’s a human pathogen that can cause a nasty list of conditions including urinary tract infections, conjunctivitis, pneumonia, and meningitis. It’s a frequent cause of hospital-acquired-infections, which is a pretty good indication that it’s a frequent cause of non-hospital-acquired-infections too (unless you keep your home more sterile than a hospital…).
Serratia marcescens can be eradicated fairly easily with the application of a bleach-based cleaning solution. Scrubbing surfaces regularly with a household cleaner should be enough to do the job. So, if you weren’t motivated to scrub down your bathrooms before, here’s some extra motivation. You’re welcome.
Incidentally, serratia marcescens has also been involved in a fascinating variety of events in human history. Before it was discovered to cause illness, it was used in “harmless” human experiments involving the transmission of disease and the effectiveness of biological warfare. Oops! Before that, it was discovered in the 19th century when it turned a bunch of Italian porridge pink. And, hundreds of years earlier, it’s possible that it was involved in a supposed medieval miracle involving blood appearing on the bread in Holy Communion. If you’re interested in those stories, the Wikipedia article is a great read.