The USA offers excellent health care. The problem is that, unless you have good insurance, it can be prohibitively expensive. It's essential to purchase travel health insurance if your regular policy doesn't cover you when you're abroad.
Bring any medications you may need in their original containers, clearly labeled. A signed, dated letter from your physician that describes all medical conditions and medications, including generic names, is also a good idea.
If your health insurance does not cover you for medical expenses abroad, consider supplemental insurance. Find out in advance if your insurance plan will make payments directly to providers or reimburse you later for overseas health expenditures.
In general, if you have a medical emergency the best bet is for you to find the nearest hospital and go to its emergency room. If the problem isn't urgent, you can call a nearby hospital and ask for a referral to a local physician, which is usually cheaper than a trip to the emergency room. Stand-alone, for-profit urgent-care centers can be convenient, but may perform large numbers of expensive tests, even for minor illnesses.
Pharmacies are abundantly supplied, but you may find that some medications that are available over the counter in your home country (such as Ventolin, for asthma) require a prescription in the USA and, as always, if you don't have insurance to cover the cost of prescriptions, they can be shockingly expensive.
Recommended items for a medical kit:
The World Health Organization publishes a superb book, called International Travel and Health, which is revised annually and is available free online at www.who.int/ith/en. MD Travel Health (www.mdtravelhealth.com) provides travel health recommendations for every country, updated regularly.
It's usually a good idea to consult your government's travel health website before departure: