Books on co-dependency are selling by the millions. Why? Because everybody has this problem to some degree.
In its extreme form, co-dependency means, for example, that John, the alcoholic, is addicted to drinking alcohol; and Mary, the co-dependent, is addicted to "helping" John (with his drinking and living problems). It also means that if their sick relationship ends--for whatever reason--John will find another Mary to "help" him; and Mary will find another John (alcoholic, drug addict, gambler, workaholic, etc.) so that she can fulfill her need to help as well as continue to suffer, blame, control, be the responsible one, hold their lives together and so on.
Co-dependents rationalize and deny that their own behavior is sick, but everybody around them knows it is. The best way to treat co-dependents is in group therapy with alcoholics, addicts and other co-dependents.
But today, co-dependents can do something about it. Reading a self-help book is a good start.