Letter 31 Tanya Lectures – Iggeres Hakodesh
Jewish unity is Jewish health. Disunity is an illness (or a symptom of it). Since Golus was brought on by disunity, exile is called an illness. [Repairing the Sinas Chinum will heal the nation and bring the geula]. This lack of unity and the sickness it causes affects the whole world, including Goyim.