http://en.wikipedia.org/wiki/I_Am_Legend_%28film%29

SPOILERS BELOW:


they shot the bridge to stop people leaving manhattan which is an island in new york - the virus started there, so they did it to try and keep all the infected people on the island... obviously it didnt work (people can use boats/swim etc)

the whole world is infected as the virus could travel in the air, and over time i assume all the air on the earth had the infection, so you got infected while breathing..

George