DED is caused by a fungus that clogs the elm tree's water conducting system, causing its leaves to wilt and the tree to die, usually within one or two seasons. It doesn't infect other tree species.
DED fungus
The fungus is spread from one elm tree to another mainly by the smaller European elm bark beetle and the native elm bark beetle. The beetles breed under the bark of dead or dying elm wood. The new generation of beetles emerge from a diseased tree. Carrying the fungal spores with them, the beetles infest healthy elms on which they feed, thereby spreading the disease.
European elm bark beetle
 
Native elm bark beetle