流行病 ≠ popular disease
popular /ˈpɑːpjələr/
adj. 收人歡迎的;喜愛的;當紅的;通俗的
popular 是一個偏向褒義的詞，
流行起來的前提是大家都喜歡的。
比如：
popular music 流行音樂
popular game 受大眾喜歡的遊戲
a popular singer 一個受歡迎的歌手
disease (疾病)是一種大家都敬而遠之的東西，
自然不能用popular修飾。
關於“流行病”該怎麼翻譯，
For the first time, the World Health Organization called the disease caused by the novel coronavirus, COVID-19, a pandemic.
世界衛生組織首次稱新型冠狀病毒COVID-19為全球大流行疾病。
pandemic /pænˈdem.ɪk/
adj. (of a disease) existing in almost all of an area or in almost all of a group of people, animals, or plants
(疾病)大規模流行的，廣泛蔓延的
例：
In some parts of the world malaria is still pandemic.
在世界上一些地區瘧疾仍在大規模流行。
n. a pandemic disease
大流行病
例：
a pandemic of influenza
流感大爆發
an influenza pandemic
流感大爆發
To understand what is a pandemic , we have to talk about what an outbreak, an epidemic and a pandemic mean.
為了理解什麼是pandemic，我們必須討論outbreak、epidemic和pandemic的意思是什麼。
outbreak
n. a time when something suddenly begins, especially a disease or something elsedangerous or unpleasant
(尤指疾病、危險或不快之事的)發作，暴發，突然發生
例：
an outbreak of cholera/food poisoning/rioting/war
霍亂/食物中毒/騷亂/戰爭的暴發
An outbreak occurs when there is an increase, often suddenly, of the number of cases of that disease, above the usual expected number of cases in a particular area.
當該疾病的病例數突然增加，超過某一特定地區通常預期的病例數時，就會發生outbreak。
epidemic /epɪ demɪk/
n. the appearance of a particular disease in a large number of people at the same time
(疾病的)流行，傳染
例：
a flu epidemic
流感的傳播
An outbreak is the term used for a more limited area, and epidemic is used for larger, more widespread populations. Epidemics are usually confined to one city or region, or even a country.
Outbreak指向的區域範圍更小，而epidemic則指範圍更廣、人口更多的地區。Epidemic通常限於一個城市或地區，甚至一個國家。
A pandemic is an epidemic that has spread over several countries and continents, and affects a large group of people.
Pandemic是一種已經蔓延到幾個國家和大洲，並影響到一大群人的流行病。
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Ref：
https://www.thestar.com.my/lifestyle/health/2020/03/12/is-covid-19-an-outbreak-an-epidemic-or-a-pandemic
https://www.npr.org/sections/goatsandsoda/2020/03/11/814474930/coronavirus-covid-19-is-now-officially-a-pandemic-who-says?t=1584066810116
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