Credit cards should be used as a back up only, although in capital cities they can be used to take cash advances from corresponding banks. Cash advances can be debited in US dollars but local currency given. Always remember to exchange only the money that you need for the country you are in, as it is often difficult to re-exchange into either US dollars or the next country's currency. Full information on health should be obtained from your travel agent or Destination Holidays, or by contacting the Medical and Vaccination centre in you capital city, as medical requirements change regularly in these areas. In most major cities this is fine, however consult your local guide.
Bottled water is also available in all areas.
However, it is unwise to eat salads or fruit that cannot be peeled, except in better known restaurants. Local remedies can be bought directly across the counter in pharmacies in South America. Altitude sickness is a non-discriminatory problem which generally doesn't affect the healthy. In most of the high altitude areas frequented by tourists, hotels have oxygen and medication is available. Remember also that radiation from the sun is more intense in the unpolluted and rarified atmosphere of high altitudes.