it's okay if you get confused by this since only real major fanboys like myself seem to get real anal about it.
the characters who are infected in this film are not dead. they are infected with a man made virus derived from the chemicals the human brain secretes when it is enraged (which has been concentrated btw.) and the ebola virus which is why the infected have bloodshot eyes and violently spew blood everywhere.
this film itself takes up 28 weeks after the original film ended and london is being rebuilt. the us military have come over from across the pond to help with keeping the controlled zone quarantined and everything is good and fine until two kids sneak out into the non quarantined part of town and discover their mother~who they thought was dead~is still quite alive and surprisingly fighting off the rage virus infection.
they take her back to the quarantined part of town for observation and all hell breaks loose.
it's got a few moments that seem to drag on forever or that really seem out of place but aside from that, was a very good sequel with an interesting subplot about biological carriers and immunity as well as a realistic ending that you could take as either there being room for another movie or something more justifiable than
"and they all lived happily ever after"