That is the question...
I've come accross quite a few questions regarding whether or not when traveling in South America you need Yellow Fever and other sickness related innoculations.
As a research sciencist in anitbody titers (shots, basically) and a fellow traveler my answer is a resounding NO. I've learned recently that practically no South American country requires a shot upon entry although some people will suggest you get innoculated against yellow fever. This is precautionary and won't hurt you, but you certainly don't HAVE to have it either. Your bodies natural immune system will protect you fairly well against anything you come accross in South America. Dengue in Brasil is perhaps some of the worst you might come accross but again, healthy adults should be fine.
As always, speak with your doctor before traveling to ensure your saftey, but don't worry if you can't squeeze shots in, before leaving.
All should be well.