After the virus made its way through the city and infected most of the population a majority of the infected would be pursuing refugees that are trying to escape. They wouldn't stop, as long as there are more people to infect and kill they are going to keep moving in that direction. The highways leading to other major population centres would have been gridlocked and most of the infected would have simply moved along those to find new food and survivors. There will still be a lot of infected in the city, but nowhere near the amount there would be if they all stayed.