Sick and sicker: immunity differences in identical twins

Jonathan and Matthew developed colds a while back.
Jonathan is on the mend now, racing the dog from the dining room through the kitchen, eating like a teenager and jumping from the sofa to the floor over and over and over again, ignoring my demands that he stop.
He's driving me crazy.
Matthew is curled up on another sofa, covered with a blanket and watching TV through half-opened eyes. It breaks my heart to watch him hold his ribs when hacking coughs overtake his body. I can't wait for the antibiotics to do their stuff.
He has walking pneumonia.
This medical inequity is nothing new.
A few weeks ago, Jonathan developed a fever that lasted for two days. Matthew caught the same virus, but his fever continued for seven days. A stomach bug that left Jonathan slightly dehydrated for a day as a baby left Matthew with bleeding ulcers and a month's prescription of Zantac.
When Jonathan develops an ear infection, Matthew often gets it in both.
My immediate reaction was to surmise that somehow, when the egg split, Matthew lost an immunity gene to Jonathan. It made sense. Matthew's always been sicker and he was born lighter, slighter, a bit more frail than his brother.
But I came to a different conclusion after doing a little research.
Recent studies are finding that epigenetics -- or the way in which genes express themselves in different environments -- is likely responsible for many differences that develop in identical twins, particularly when it comes to immunity.
Sometime after conception, either in the womb or outside it, one of the twins was likely exposed to a virus or bacteria that missed the other. In fighting off the intruder, he either gained any army (Jonathan), strengthening his physical fortress, or lost one (Matthew), weakening his defenses.
Their identical genes learned to express themselves in different ways when confronted by bacterial infections or viruses, resulting in permanently different immune systems.
Amanda Carpenter, a virology student, writes about an excellent example here. Identical twins born in 1983 were exposed to HIV, the virus that causes AIDS. Fifteen years later, one was relatively healthy and strong while the other was sickly and ill.
After studying blood samples from the pair, researchers concluded that environment, which led to a depressed immunity system in one twin, was likely to blame for their different reactions. Their shared DNA did not ensure a shared prognosis.
My hope for Matthew is that someday this will turn itself around -- that someday he'll be exposed to something that makes him stronger instead or weaker -- and that Jonathan's immunity will remain unchanged.
Who knows?
Maybe this is the one.
Maybe all that hacking and coughing that kept me up last night, worried that he will choke or stop breathing, is the just influence his genes needs.