The American health care system is world-class in quality, but can be very expensive. Americans generally use private health insurance, paid either by their employer or out of their own pocket; some risk paying high hospital bills themselves, or depend on government subsidized health plans. As a traveler you should have travel insurance or you will potentially face high costs if you need medical care.
Most metropolitan areas will have a mix of public and private hospitals, and in turn, U.S. private hospitals can be either nonprofit or for-profit. Public hospitals located in wealthy suburbs can be as good as private ones, but in poorer inner-city areas, public hospitals are usually overcrowded and run-down and should be avoided by tourists. However, many public hospitals are also the Level I regional trauma centers for their respective metro areas (i.e., they guarantee 24-hour on-site availability of all major types of medical specialists), which means that you will be taken there if critically injured.
In a life-threatening emergency, call 911 to summon an ambulance to take you to the nearest hospital emergency room ("ER"), or in less urgent situations get to the hospital yourself and register at the ER's front desk. Emergency rooms will treat patients without regard to their ability to pay, but you will still be presented with a bill for all care. Do not use ERs for non-emergency walk-in care. Not only can this be 3-4 times more expensive than other options, but you will often wait many hours (or days) before being treated, as the staff will give priority to patients with urgent needs. In most areas, the charge for an emergency room visit starts around $500, in addition to any specific services or medications you may require. Most urban areas have minor emergency centers (also called "urgent care", etc.) for medical situations where a fully equipped emergency room would be excessive, such as superficial lacerations. However, their hours may be limited, and few are open overnight.
Walk-in clinics are another place for travelers to find routine medical care, letting patients see a doctor or nurse-practitioner without an appointment (but often with a bit of a wait). They are typically very up-front about fees, and always accept credit cards. To find one, check the yellow pages under "Clinics", or call a major hospital and ask. Make sure to tell the clerk you will be paying "out of pocket"; if they assume an insurance company will be paying for it, they may order tests that are not medically essential and in some cases bill for services that aren't actually provided.
Dentists are readily available throughout the United States (again, see the yellow pages). Dental offices are accustomed to explaining fees over the phone, and most will accept credit cards.
Most counties and cities have a government-supported clinic offering free or low-cost testing and treatment forsexually transmitted diseases; call the Health Department for the county you are in for more details. Many county clinics offer primary health care services as well, however these services are geared towards low-income residents and not foreign travelers. Planned Parenthood (1-800-230-7526) is a private agency with clinics and centers around the country providing birth control and other reproductive health services for both females and males.