Within what's left of the cities, cholera from unclean water and typhoid from unburied corpses would be rife. With radiation poisoning compromising everyone's immune systems, it could be anything that kills people. I don't think true epidemics would occur though, as population density would simply be too low to make them realistic, unless you've got huge refugee camps in one place, in which case we're back to cholera and typhoid.
Malnutrition and starvation are probably going to be your big killers, after direct radiation poisoning.