Short answers, pulled from the story.
Addiction is a neuropsychological disorder characterized by a persistent and intense urge to use a drug or engage in a behavior that produces an immediate psychological reward, despite substantial harm and other negative consequences. It is understood as a brain disorder shaped by both psychosocial and neurobiological factors.
Addiction centers on the brain's reward system, specifically the mesolimbic or mesocorticolimbic pathway. Dopaminergic neurons in the ventral tegmental area release dopamine into the nucleus accumbens, and virtually all addictive drugs increase dopamine release along this pathway. The prefrontal cortex, amygdala, and hippocampus are also involved.
ΔFosB is a gene transcription factor and a common factor in virtually all forms of behavioral and drug addiction. Its overexpression in the D1-type medium spiny neurons of the nucleus accumbens drives the neural and behavioral changes of addiction, and it is used preclinically as an addiction biomarker.
Genetic and environmental risk factors each account for roughly half of an individual's risk of developing an addiction. Genetic factors account for 40 to 60 percent of the risk for alcoholism, with similar heritability indicated for other drug addictions.
In adolescence the incentive-reward systems in the brain mature before the cognitive control center, so reward sensitivity is high while inhibitory control lags. Most individuals first use addictive drugs as teenagers, and over 90 percent of people with an addiction began drinking, smoking, or using illicit drugs before age 18.
Behavioral addictions are compulsions to engage in a natural reward such as gambling, gaming, shopping, eating, or sex despite adverse consequences. The DSM-5 and ICD-10 recognize only gambling as a behavioral addiction, while the ICD-11 also recognizes gaming addictions.
The word addiction derives from the Latin addico, meaning giving over, with both positive connotations of devotion and negative ones of being enslaved to a creditor under Roman law. In the 19th century, temperance movements narrowed its meaning to a drug-related disease.