Next year at the World Cup, outbreaks of dengue fever are possible. People in at-risk areas should take measures to prevent mosquito bites, and support public health measures to decrease the mosquito population.
Many foreigners have fear of contracting yellow fever in Brazil but the disease now is rare in Brazil. An effective vaccination is available, and the WHO has just revised their recommendations such that at-risk individuals only receive one shot in their lives. Booster shots are no longer necessary.