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Heat is the function of Fire, and Fire is the substance of Heat. Therefore, when speaking of Fire, one does not speak of Heat; when speaking of Heat, one does not speak of Fire. They are one yet two, and two yet one. "Warmth" (Wen) refers to a state where heat is not intense. In the theory of the Six Qi (Wind, Cold, Summer-heat, Dampness, Dryness, Fire), only Fire is mentioned, not Warmth, using "Fire" to encompass both. The term "Warmth-Heat" (Wen-Re) is a general designation for Warm Diseases (Wenbing) and Heat Diseases. When the pathogenic factor is mild, it is called Warmth; when severe, it is called Heat. In modern times, "Warm Disease" is generally used as an umbrella term, which actually encompasses etiologies such as Heat, Summer-heat, Dampness, and Dryness.

Warm diseases and Summer-heat diseases caused by Winter Cold injury manifesting in Spring or Summer, as well as diseases contracted in Spring or Summer but manifesting in Autumn or Winter, are all referred to as Latent Pathogens (Fuqi). Those that manifest immediately upon contraction are referred to as New Contractions (Xingan). New contractions involve milder pathogens and are relatively easier to treat; latent pathogens involve heavier pathogens and are more difficult to treat. Therefore, the methods for treating Warm Diseases differ not only from those for Cold Damage (Shanghan), but Latent Pathogens and New Contractions cannot be compared on the same level.

Plague (Piyi) was originally named Liyi, referring to infectious diseases. *Suwen: Cifa Lun* states: The Yellow Emperor asked: I have heard that when the Five Plagues arrive, they are all contagious, regardless of age, and the symptoms are similar. *Suwen: Liuyuan Zhengji Dalun* states: When the Malignant Qi (Li) arrives in force, the people are prone to sudden death. These texts describe the symptoms of Liyi. *Shuowen Jiezi* explains: Li is a malignant disease; Yi is a disease affecting the common people. The character derives from the phonetic component of Yi (service). *Shiming* explains: Yi is synonymous with Yi (service/役). It implies ghosts performing service. In *Cifa Lun*, the distinction between Li and Yi is deduced through Heavenly Stems and Earthly Branches, stating that Heaven generates Yi, and Earth generates Li. This suggests that the difference between Yi and Li lies merely in the upper/lower and rigid/flexible aspects. Thus, they are categorized under the Five Elements (Metal, Wood, Water, Fire, Earth), known as the Five Plagues. However, in actual practice, both Li and Yi have Cold and Warm varieties. Among them, Warm Li and Warm Yi are more prevalent, while Cold Li and Cold Yi are less common. *Liuyuan Zhengji Dalun* states: Their illness is Warm Li, spreading widely, affecting distant and near areas alike. It also states: Their illness is Warm Li. These references mention Warm Li without mentioning Cold Li, likely using Warm Li to generalize Cold Li. *Laozi* states: After a year of disaster, there will inevitably be Warm Plague. *Baopuzi* states: One who has passed through a Warm Plague area fears nothing. Both texts mention Warm Plague without Cold Plague, again using Warm Plague to generalize Cold Plague. Later generations established the term "Miscellaneous Plague" (Zayi) to categorize complex syndromes within epidemics.

Since the etiology and treatment methods for Warm Diseases and Plagues are distinct, ancient scholars distinguished them clearly, and they should not be confused. Fearing confusion, later generations changed the character "Wen" (Warm) in "Wenyi" (Warm Plague) to "Wen" (Plague/Wen). Wu Youke failed to understand this meaning. Noting the absence of the character "Wen" (Plague) in ancient texts, he erroneously conflated Warm Diseases and Plagues into a single entity. (Wu Youke stated: *Shanghan Lun* says that fever with thirst and no aversion to cold constitutes a Warm Disease; later generations added the radical to create 'Wen', which is Plague. Essentially, the character 'Wen' did not exist in antiquity; it was a later alteration. One should not treat 'Wen' and 'Wen' as two different diseases based on this textual change.) Those who do not understand Warm Diseases view "Wen" as the disease name itself. For Warm Plague and Warm Li, "Yi/Li" is the disease name, specifying that it is the Warm type of Plague or Li. Precisely because the character "Wen" (Plague) did not exist in ancient times, we know that the "Wen" in Wenyi and Wenli is not a disease name. How can one then erroneously conflate Warm Disease with Liyi?

When discussing matters, one must understand flexibility and adaptation; medicine is no exception. Illnesses resulting from contracting Spring Warmth, Summer Heat, Autumn Coolness, or Winter Cold are considered normal (Chang). *Suwen* states: Extreme Yin transforms into Yang; extreme Yang transforms into Yin. Winter Cold injury leading to Spring Warm Disease, Spring Wind injury leading to Summer Sundry Diarrhea (Sunxie), Summer Heat injury leading to Autumn Malaria, and Autumn Dampness injury leading to Winter Cough are all examples of Latent Pathogens, termed "Variation" (Bian). However, these are still non-contagious and belong to the "normal within variation." Conversely, infectious diseases such as Throat Sha (Housha), White Throat (Baihou), Cholera (Huoluan), Hidden Yin Syndrome (Fuyinzheng), Sha Syndrome (Shazheng), Dysentery (Liji), Epidemic Spasm (Yijing), and Bubonic Plague (Shuyi) represent the "abnormal within variation." (Note: Baihou, Huoluan, Shazheng, and Liji can also occur non-infectiously, in which case they fall under the "normal within variation.") Therefore, to thoroughly investigate the origins and details of Warm Diseases, one must comprehend the principles of Normalcy and Variation.

However, since Wu Youke conflated Warm Diseases and Plagues, many have followed suit, obscuring the principles of Normalcy and Variation. Books on treating Plagues often mix in Warm Disease content, and books on treating Warm Diseases often include Plague content. Subsequent scholars, unless they are diligent and deep thinkers, are rarely unaffected by this confusion. (Songru) deeply worries about this and has therefore critically reviewed books concerning Warm Diseases and Plagues separately, enabling future readers to make informed choices.