You might need a B-2 Visa

There are several websites that explain the process for applying for a medical visa, e.g. http://www.usavisitorvisa.com/medicalvisa/medical_visa.htm and http://travel.state.gov/visa/temp/types/types_1262.html 

Present US immigration policy presumes that every visitor visa applicant is an intending immigrant (has an intention to stay back in the U.S. permanently). Therefore, applicants for visitor visas must overcome this presumption by demonstrating that:

1. The purpose of their trip is to enter the U.S. for business, pleasure, or medical treatment.
2. That they plan to remain for a specific and limited period.
3. Evidence of funds to cover expenses in the United States;
4. Evidence of compelling social and economic ties abroad; and
5. That they have a residence outside the U.S. as well as other binding ties which will insure their return abroad at the end of the visit.

You might be eligible for a visa waiver

If you intend to visit the United States for 90 days or less, unders certain circumstances, you might be eligible for a visa waiver. To learn more about the prograsm and requirements, check http://travel.state.gov/visa/temp/without/without_1990.html

You will need to satisfy certain immigration requirements even if you are not the patient

For example , suppose you want to accompany a family member while they get care. Or, what if you intend to donate your kidney to a family member who is having a transplantation operation in the US? According to http://www.immihelp.com/visitor-visa/visitor-visa-usa-medical-treatment.html, you will need the necessary approval from US immigration officials before your US arrival.

If you plan to come to the US for care or plan to accompany someone who is, make sure you have a visa or documentation of a visa waiver.  Prepare ahead and don’t procrastinate. Getting approval can take months and you should give yourself enough time to get it.