You are likely to acquire your friend's anxious nature; your immunity may weaken and you may suffer from seasonal flu more often if your close friend has low immunity; similarly, chances of you being obese are high if your friends are overweight.  

A new study published in PLOS recently, says that daily interactions between individuals can have both negative as well as positive impact on their health. "The study sheds light on an important component of the genetic architecture of complex traits, one that lies outside the individual, in social partners. Social genetic effects have already been shown to play an important role in artificial selection of livestock and have important evolutionary consequences. Our results provide evidence that SGE are also an important component of health and disease."

The effect or influence of your interactions is higher in cases of anxiety, wound healing, immune function, and body weight. The study states that social genetic effects explained up to 29% of phenotypic variance, and for several traits their contribution exceeded that of direct genetic effects.