Medical insurance is important to have at all times.  While traveling, it's doubly important, and you should make sure that you ahve adequate insurance for your trip.

In general, medical care in Brazil is far cheaper and doctors themselves must know Portuguese, English and a third language proficiently. 

Make sure that your current health provider will cover you in Brazil and make sure you know how to contact them from Brazil in the event you need medical attention!  Don't wait until you need medical attention and then start looking for this information.

If your medical coverage does not include trips to Brazil, it is recommended to buy travel insurance at the very least.

• While traveling, you are more likely to take on risky behavior.  Trying out some new water sport, running along the beach, climbing some rocks.
• You're in an unknown environment, which can greatly increase your chances of having an accident.
• You're body might run into some new water borne disease, while not life threatening directly, could lead to dehydration and other side issues.  While the water is very good Brazil, it's hard to tell if you're going to be susceptible to something in their water supply.  It happens, it goes away within a few days, but it's your vacation and you don't want to ruin it.  
• Make sure you've got at least your hepatitis B and C shots.  Your local travel clinic can give them to you, but they do take upto 6 months to take effect and require 3 shots.  Initial shot, a second shot a month later and a third shot 6 months later.  There is a way to speed up this process where it takes roughly 30 days, but isn't as effective.  It's always recommended to get these shots when traveling anywhere in South America.  So get them early.
• Depending on the specific area you are traveling to, you should ask your travel clinic if they recommend any other shots.