If you are an American, you do need a visa to enter Brazil. Whoever told you do not is in error. The visa is good for 90 days after you get it, so don't apply too soon. Your travel agent may be able to help you with this.
If you are traveling to Brazil from or through any place where yellow fever is endemic, you must have a yellow fever immunization as well. This would include Peru and Bolivia.
Yellow fever does exits in Brazil, primarily in the Amazon area, although it is also found around Iguassu Falls (as well as dengue fever). Although when I went there it was not required, I decided to get it anyway as a protection for myself. I also have had both hepatitis A and B immunizations (also suggested for all foreign travel). Better to be safe than sorry. There is no immunization for dengue fever. Be sure to wear long sleeves shirts, slacks and use a good quality mosquito repellant in any tropical areas.