robert only confirmed the existence of airborne version of the virus after the mass infections. when he went to pick up his wife he only suspect that it might be airborne not yet confirmed. by then the infection caused by the non airborne version of the virus is already causing chaos so one way of stopping the virus is to completely isolate those infected on the island hence the bridges are blown up. not knowing that they might have transported humans infected by the airborne strain over.i think i saw sumthing like the virus killing them by causing massive internal bleeding or sumthing like that. 90% gonna get killed, less than 1% are naturally immune and the rest turns into darkseekers.
he also mentioned that k9 are immune to airborne strain of the virus. i think all animals are immune to airborne strain but not the strain that are transfered by bite. that's why there are lions and deers around hehe.