The points about their being more money involved it it is a disease are right on, IMHO. And since there are s man more addicts that say, legionairre's disease guess where money gets funnelled?
That said, you don't have to "catch" a disease like malaria or the flu. Many diseases people are just born with, and they experience no symptoms until the right set of circumstances triggers the disease. Addiction is one f tha latter, IMHO.