Dengue Fever is one of the biggest killers in tropical countries. It’s carried by mosquitoes that have proven tough to eradicate, so now officials in Brazil are trying a new approach: mosquitoes that have been genetically modified.Dengue Fever is one of the biggest killers in tropical countries. It’s carried by mosquitoes that have proven tough to eradicate, so now officials in Brazil are trying a new approach: mosquitoes that have been genetically modified.