Literally, a cesspool is place underground where sewage gathers, but figuratively, you might hear anything that's full of germs or just plain disgusting described as a cesspool. Either way, you'll want to steer clear.
Think of a cesspool as an underground swimming pool full of nastiness, which of course rhymes with cess. But the word cesspool isn't just used to describe a storage place underground where sewage is held temporarily. It's also a term you might use to negatively describe something that's swimming in germs or corruption. Stuck on a long flight between two people with a cold? That's a cesspool of germs right there for you.