San Diego is a city in the state of California located south of Los Angeles. It was only mentioned in passing. When the remaining members of a National Guard unit flee from the West View Library after been overrun, they said that their destination was San Diego.
San Diego was mentioned again during Season 2 as a possible destintion for the survivors aboard The Abigail. However, George Geary mentions to Travis that San Diego was bombed by the military. This action seems to fit with the overall aims and objectives of Operation Cobalt, as the military were trying to stem the flow of infected streaming out of major urban centers.
It is currently unknown what effect the bombing of San Diego has had on the Mexican city of Tijuana, given the shared boarder crossing between the two cities.