"The cities are aliiivveee with the sound of music..."
Oh come on, I know I can't sing - its a joke - for the love of - fine! Some people just think you need to be serious about these things. Most people, really. But the big things about the cities, is that you absolutely must be flexible in your expectations, because there can be almost anything, and always something.
Even the deadest city is very much alive. Its just too convenient a location to ignore. From the standpoint of just about any any animal, human, rat, or feral dog - there are plenty of places to hide from predators, or at least escape bad weather for a while. A lot of the formerly domestic ones are simply used to the area, and others have learned about all the neat ambush locations the alleyways provide. The overhangs and tunnels are great for bats, parks for forest critters, and many escaped zoo animals haven't wandered that far from where they were contained.
On top of that, there are still going to be people. You might not see them everyday, but that is because they're intentionally hiding from you - and if you're smart, you'll hide from them. There are resource collectors of course, outlaws, survivor outposts that have taken over factories, and transients who think a brick facade is safer than a tent. Some cities are even almost repopulated - not metropolises, but you'll see trade, or gangs holding territory, and bazaars, and even some concentrated efforts to take and hold buildings of cultural importance - it takes a lot of work to keep a museum safe. (Wouldn't you just love to camp with the dinosaurs in the natural history museum? Really - who wouldn't?)
Of course, cites also seem to be the default habitat of reanimates. You're not likely to see advanced type threes in the middle of nowhere, but stick around the city long enough, and chances are you'll be killed by one. No, I don't mean encounter one - killed is the operative word here. Those things know how to navigate the city too, weather it means popping out of the old subway station, or leaping from the third story of an office building onto your head, or beating you to death with a stop sign out in the open - they can and will use the streets to your disadvantage.
Returning to the opening chorus - its the signs of life that are the worst part. Towns might be totally depopulated and creepy because of that. Old factories can have tetanus and toxins all about. Bandits may or may not be lying around the next bend of the road. In the city, you know there is something out there that bears ill will, its just a matter of time, and no amount of caution or medicine is really going to prevent that.