We know, of course, that colds and flu and other illnesses are contagious. When a cold is going around, we remind our children to wash their hands, avoid touching their eyes and mouths, and keep a safe distance from other children who are sick.  We may also encourage an increased intake of vitamin C, the use of antibacterial gels and other preventative measure. For the most part, we also trust that other parents will keep their sick children home to avoid the spread of illness. If there is a serious outbreak of illness, we may even keep our child home from school for a period of time.
When our children become teens, we teach them about sexually communicable diseases and how to avoid these through abstinence or protection. In short, our response to contagious illnesses is to remove the sick person from contact with others, remove ourselves from contact with the sick person, and/or take specific practical measures to protect ourselves from the germs or virus that cause the illness.
But physical diseases are not the only problems that are communicable. Negative behaviors can be passed from one person to another, though these problems are socially rather than physically contagious. Some self-destructive teen behaviors, in fact, are extremely socially virulent and can spread among groups of teens as rapidly and destructively as a physically communicable disease.
Socially contagious behaviors include bullying; dangerous games such as the choking game, high-risk dares, and drinking games, hitting games, etc.; promiscuity or high-risk sexual behaviors; eating disordered behaviors such as binging, chronic overeating, purging or restricting; drug and alcohol abuse; cutting and other forms of self-harm; and almost any other negative behavior that occurs with frequency among adolescents.
It may be useful to understand that contagious behaviors spread in much the same way as contagious illnesses (they are passed from one person to another through exposure).  When we know that someone is carrying a contagious disease, we remind our family members of the risks of exposure, we give them tips regarding how to avoid infection, and (when necessary) we remove them from contact with the contagious person(s) or circumstances. If our child becomes infected, then treatment becomes necessary.
Sometimes similar measures are in order if behaviors like those described above seem to be an issue in our teen’s social circles. Education and voluntary avoidance are the first steps to take, but when those strategies prove inadequate, we may need to actually change our child’s social circumstances completely, removing her from dangerous situation entirely.