Can't give a good answer here.  Can't determine anything from the pictures as to how close the trees are growing to each other.  If the elm had been affected by Dutch Elm disease initially, it would have died outright.  No, trees do not develop resistance as do humans.  There are cultivars of elms on the market now that are resistant to Dutch elm disease but they have been bred for the resistance.  It will be a judgement call on your part that could be helped by an onsite visit by a Certified Arborist, one who specializes in everything woody.  You can find one or more Certified Arborists in your zip code at www.treesaregood.com  Click on the Find an Arborist tab.  Arborist evaluations are not expensive and are the best way to help  arrive at difficult decisions.  

Good luck!