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Herd immunity, generated from previous exposure to cold viruses, plays an important role in limiting viral spread, as seen with younger populations that have greater rates of respiratory infections. Poor immune function is a risk factor for disease. Insufficient sleep and malnutrition have been associated with a greater risk of developing infection following rhinovirus exposure; this is believed to be due to their effects on immune function. Breast feeding decreases the risk of acute otitis media and lower respiratory tract infections among other diseases, and it is recommended that breast feeding be continued when an infant has a cold. In the developed world breast feeding may not be protective against the common cold in and of itself.