The social contagion researchers who have steadily brought us research on obesity, quitting smoking and happiness contagion have now fresh evidence of the contagion of another behavior: loneliness.

Although it seems counterintuitive that people who feel alone – even when surrounded by other people – can infect people instead of dropping out of sight, lonely people tend to influence people in their web with their feelings of aloneness. Although everyone averages 48 days of loneliness each year, having a lonely friend causes you to add 17 more days a year.

Not only do lonely people impact your own feelings, they can cause your entire social network to disintegrate. “If you’re lonely, you transmit loneliness, and then you cut the tie or the other person cuts the tie,” Dr. Nick Christakis, a Harvard Medical School authority on this topic says. “But now that person has been affected, and they proceed to behave the same way. There is this cascade of loneliness that causes a disintegration of the social network.”

Loneliness is becoming a national epidemic. In 1985, surveyors asked people how many confidants they had on average, and the answer was three. That same question now yields the answer “none” for at least 25% of those sampled. And by the year 2010, 29 million people will live alone.

As a coach, I’ve been fascinated by the huge number of clients who report tremendous professional success, but profound friend trouble. Many people profess to not know how to stay in touch with old friends, or find the time to nurture current relationships. They have discovered that a few hours on Facebook or LinkedIn every week is a poor substitute for actually sitting down with someone over a shared meal, but they fumble in these attempts, too, and second-guess themselves about their friend skills constantly.

Dale Carnegie offered all of us a simple set of prescriptive behaviors in his time-tested book, “How to Win Friends and Influence People” over fifty years ago. Smile. Remember someone’s name. Compliment others genuinely. Don’t whine, condemn or criticize. Be a friend to others. His advice is as relevant now as it was when he wrote it, but possibly more urgently needed than ever.