there is potential for aedes ypti, the me vector uld ansmit ylow fevein citie whicwill be very complicated. reporter: one in five cases of yellow fever proves fatal. healthlinics arelready at bre. theris a yellofever vacce, but ly 60% of azilians a vaccinated. reporter: overcrowding is a serious problem in the favelas of rio. defective water tanks and overflowing rubbish dumps are ideal breeding grounds for mosquitoes. wthe mosquitoes take longer to. grow. even so, it doesn't take gabriel sylvese long to nd some larvae. gabriel: we should be warned about. ichikunnya, more read, like ha. we canave deng, zika, so it is even worse than to have just during the summer. repoer: local shop and hou owners do their bit for the project. the mosquito traps scattered abt the cityre emptied regulars later examined in the lab. tinfected mosquitoes have beend released io the envinment, wolbachia bacteria end up present in the bulk of the mosquito population. now, the scientists have to wait until the next epidemic of mosquito-borne diseas in humans to see if there is a reduction in infection rates in tho