A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that are associated with specific signs and symptoms. A disease may be caused by external factors such as pathogens or by internal dysfunctions. For example, internal dysfunctions of the immune system can produce a variety of different diseases, including various forms of immunodeficiency, hypersensitivity, allergies and autoimmune disorders.

रोग अर्थात अस्वस्थ होना। यह चिकित्साविज्ञान का मूलभूत संकल्पना है। प्रायः शरीर के पूर्णरूपेण कार्य करने में में किसी प्रकार की कमी होना 'रोग' कहलाता है। जिस व्यक्ति को रोग होता है उसे 'रोगी' कहते हैं। हिन्दी में 'रोग' को 'बीमारी', 'रुग्णता', 'व्याधि' और 'विकार' भी कहते हैं।