The Economist recently published an article about the relationship between loneliness and health. As you may have guessed, lonelier people are less healthy. The authors of the study even identified a pathway through which this may operate: a decreased activity in the virus-fighting genes in the lonely people and an increase in the bacteria-fighting genes. The latter can lead to a chronic inflammatory response if loneliness is chronic, subsequently reducing your chances of survival.
Let’s put aside the possibility of reverse causality (healthier individuals probably tend to be more social). The hypothesis that loneliness has a causal effect on health is a plausible one, but something doesn’t add up here. The authors claim that the regulation of genes is affected by the extent of your "loneliness”. In the modern world, loneliness is not necessarily the same as being alone. Most people who live in big cities (or even small cities) encounter many people during the day. So you would think those people would affect your gene expression as well.
The study, however, measured loneliness by the quality of your social relationships (actually, because it’s a meta-analysis, it’s really not clear how they measure it). Can it really be that your gene expression is affected by the characteristics of your personal relationships in addition to by how many people you encounter day-to-day? I don’t think we have such a fine-tuned makeup, personally, unless one wants to make the claim that the effect works through hormones that are only released during personal interactions with people.
My hypothesis would be that the key component here is stress (in addition to reverse causality). Someone who is classified as "lonely” still has relatives and other people in their lives. If you don’t have "close” relationships, chances are you have non-close relationships that stress you out, leading to a decline in health.
Here’s a link to the original article. The authors are actually very careful about not over-interpreting their findings, unlike the Economist.