 For example, a plant species may be highly susceptible to a particular plant disease, meaning that exposed populations invariably become extinct or decline heavily.However, that species may not be vulnerable if it occurs only in areas where exposure to the disease is unlikely, or if it occurs over such a wide distribution that exposure of all populations is unlikely.Conversely, a plant species may show low susceptibility to a disease, yet may be considered vulnerable if the disease is present in every population.