There was an interesting study of over 12,000 people published in the New England Journal of Medicine recently (Christakis and Fowler, 2008) looking at the way that stopping smoking seems to help those around us stop smoking as well.  So if I stop, the chances of my spouse smoking decrease by 67%, of a friend smoking decrease by 36%, of a co-worker (in a small firm) by 34%, and of a sibling smoking by 25%.  That's a great set of effects/associations.

Doubly interesting though that the reverse effect also occurs - unhealthy patterns also seem to be 'infectious'.  So the same authors showed in a paper published last year (Christakis and Fowler, 2007) that a person's chances of becoming obese "increased by 57% if he or she had a friend who became obese in a given interval.  Among pairs of adult siblings, if one sibling became obese, the chance that the other would become obese increased by 40%. If one spouse became obese, the likelihood that the other spouse would become obese increased by 37%.  These effects were not seen among neighbors in the immediate geographic location. Persons of the same sex had relatively greater influence on each other than those of the opposite sex. The spread of smoking cessation did not account for the spread of obesity in the network."

Fascinating!  There are both personal and public health messages here.   

Christakis, N. A. and J. H. Fowler (2008). "The collective dynamics of smoking in a large social network." N Engl J Med 358(21): 2249-2258.  [Abstract/Full Text]
Christakis, N. A. and J. H. Fowler (2007). "The spread of obesity in a large social network over 32 years." N Engl J Med 357(4): 370-9.  [Abstract/Full Text]