Most states require dogs to be vaccinated against rabies, which is rare but not unheard of in domestic animals. It is almost always fatal in humans.
Usually, you must have proof that your dog has an up-to-date rabies vaccination to get a dog license. Vaccines that last for three years are available for dogs more than four months old, making compliance easy. Many cities offer low-cost vaccinations at permanent clinics (such clinics usually offer spaying and neutering for a reduced fee as well), or special one-day clinics where owners merely have to show up to get pets vaccinated. Washington, D.C., for example, holds an annual clinic at which dogs can get free rabies vaccinations.
Don't think your dog is safe from rabies because you live in a city and rarely come into contact with wild animals. Common species of "urban wildlife"—skunks, raccoons, and bats—can spread the disease to pets. Healthy wild animals usually avoid domestic animals, but sick ones may not, and they are also more likely to be out in the daytime. Feral cats may also spread the disease.
Some cities impose additional vaccination requirements—for example, you may be required to get your dog immunized against distemper, a relatively rare but very contagious and usually fatal disease in dogs.
You may need to have proof of recent vaccinations for your dog before you can take it into another state or country.