Mainstream smoke is that which is caused by the lighting of the cigarette, and side stream smoke is the smoke inhaled by the smoker. Secondhand smoke is not more dangerous that mainstream or side stream smoke. All three are equally dangerous, according to the American Lung Association. The difference is in the frequency and intensity of exposure for an individual. There's no safe level of exposure to Group A carcinogens.
Secondhand smoke is the third leading cause of preventable death in the United States.
What you may be referring to in your question regarding the extra dangers of secondhand smoke concerns young children. Babies and children are especially vulnerable to secondhand smoke. According to the American Lung Association, between 150,000 and 300,000 lower respiratory tract infections in infants and children younger than 18 months each year are due to exposure to secondhand smoke, leading to as many as 15,000 hospitalizations.