http://www.sciencedaily.com/releases/2007/09/070913081048.htm This is a most interesting article as to how they were able to correlate loneliness to the functioning of immune system.
Hmm I would think that people with a family will find more pressure to live up to their expectations and make sure the family gets everything they need thus affecting their health in some way due these worries.
there's also some physiological changes other than the gene expression as well. loneliness can lead to depression and there's evidence showing that in depressed peoples there's an increase in size in one of the parts of the brain (hypertrophy of the hypothalamus.. i think... can't remember) that produce a hormone called cortisone, which is a corticosteroid which reduces the inflammatory response. therefore... more susceptible to infections and other crap that ain't so great for your body