**1.5 Co-infection**

Natural co-infection with dengue virus can occur in highly endemic areas where several serotypes have been transmitted for many years. Cases of simultaneous infection by more than one species of arboviruses in mosquito or human host were reported (Meyers& Carey, 1967; Gubler et al., 1985). In Brazil, one case of co-infection by DENV-1 and DENV-2 was reported in the patient with classic dengue fever (DF) from Southeastern region through immunofluorescence and RT-PCR (Santos et al., 2003). Another case of co-infection by DENV-2 and DENV-3, was observed in 2005 in the Northeastern region from a patient with DF (Araújo et al., 2006). Simultaneous infection by different strains of dengue virus in mosquitoes and humans underscores the potential for recombination (Santos et al., 2003). Although recombination is rarely recorded in positive-stranded RNA viruses (Lai, 1992), recombination occurrence in picornavirus, coronavirus, and alphavirus have been suggested. The latter viruses are also transmitted by mosquitoes (Hahn et al., 1998.Variations in dengue virus and the occurrence of co-infections with different DENV serotype may lead to genetic exchange between strains increasing the likelihood of recombination (Kuno, 1997).

During an outbreak of dengue in São José do Rio Preto, State of São Paulo, 365 samples were positives to DENV-3, 5 samples were to DENV-2, and 8 to Saint Louis encephalitis flavivirus (SLEV). Among the positive samples, one co-infection was detected for DENV-2 and DENV-3. Co-infection of each distint DENV serotype or other flavivirus during dengue outbreaks seems to be common (Terzian et al., 2010).
