Unfortunately, it means that disease transmission is a real issue in city and other high traffic dog-walking environments. Many fecal parasites can survive in the environment and be infective for weeks to months. Whipworms can survive in soil for up to 7 YEARS! Parvovirus is a very serious and possibly fatal viral infection that is transmitted from fecal material, and can survive in soil for up to a year. Some of the parasites that our dogs can pick up, such as Giardia, can then be passed on to us - these are considered 'zoonotic'.
Does this mean that we should all keep our dogs on lockdown? Concrete all the green space we can find? Pour bleach on the ground in front of our dog's feet? Of course not! What fun would that be? It does mean that veterinary care in the city is of the utmost importance! Keeping your furry friend vaccinated against Parvo (usually included in the Canine Distemper combination vaccine) is almost 100% effective when administered appropriately. If you have a new puppy, it is important to be cautious until they are considered fully vaccinated at 16 weeks of age. There is a waxing/waning effect with protective antibodies from a puppy's mother vs. the vaccinations administered, so it is important to limit your puppy's exposure to the outside world until your vet gives you the okay.
What all of this means is that you can still take your dog for a walk, to play in the park, or let them roam the streets of Philadelphia off-leash....wait, scratch that last one! Since you are not going to keep your dog on lockdown, make sure that you're taking the necessary precautions to keeping the *real* yucky stuff out of their poo.