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   Lonely people, it seems, are at greater riskthan the gregarious of developing illnesses associated with chronicinflammation, such as heart disease and certain cancers. A paper published lastyear in the Public Library of Science, Medicine, shows the effect on mortalityof loneliness   is comparable with thatof smoking and drinking after examining the results of 148 previous studies andcontrolled for factors such as age and pre-existing illness.

Steven Cole of the University ofCalifornia, Los Angeles, thinks he may know why this is so. He told theAmerican Association for the Advancement of Science meeting in Washington,D.C., about his work studying the expression of genes in lonely people. Dr.Cole harvested samples of white blood cells from both lonely and gregariouspeople. He then analysed the activity of their genes, as measured by theproduction of a substance called messenger RNA. This molecule carries    instructions from the genes telling a cellwhich proteins to make. The level of messenger RNA from most genes was the samein both types of people. There were several dozen genes, however, that wereless active m the lonely, and several dozen others that were more active.Moreover, both the     less active andthe more active gene types came from a small number of functional groups.

   Broadly speaking, the genes less active in thelonely were those involved in staving off viral infections. Those that weremore active were involved in protecting against bacteria. Dr. Cole suspectsthis could help explain not only why the lonely are iller, but how, inevolutionary terms, this odd state of affairs has come about.

    The crucial bit ofthe puzzle is that viruses have to be caught from another infected individual andthey are usually species-specific. Bacteria, in contrast, often just lurk inthe environment, and  may thrive on manyhosts. The gregarious are therefore at greater risk than the lonely ofcatching  viruses, and Dr. Cole thussuggests that past evolution has created a mechanism which causes white cellsto respond appropriately. Conversely, the lonely are better off ramping uptheir protection  against bacterialinfection, which is a bigger relative risk to them.

     What Dr. Cole seemsto have revealed, then, is a mechanism by which social environment reachesinside a person's body and tweaks its genome so that it responds appropriately.It is not that the lonely and the gregarious are genetically different fromeach other. Rather, their genes are regulated differently, according to howsociable an individual is. Dr. Cole thinks this regulation is part of a widermechanism that tunes individuals to the circumstances they find themselves in.

What can we know about viruses and bacteria from the fourth paragraph?