流行病 ≠ popular disease
popular /ˈpɑːpjələr/
adj. 收人欢迎的;喜爱的;当红的;通俗的
popular 是一个偏向褒义的词，
流行起来的前提是大家都喜欢的。
比如：
popular music 流行音乐
popular game 受大众喜欢的游戏
a popular singer 一个受欢迎的歌手
disease (疾病)是一种大家都敬而远之的东西，
自然不能用popular修饰。
关于“流行病”该怎么翻译，
我们先看看世卫组织的声明
For the first time, the World Health Organization called the disease caused by the novel coronavirus, COVID-19, a pandemic.
世界卫生组织首次称新型冠状病毒COVID-19为全球大流行疾病。
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)大规模流行的，广泛蔓延的
例：
In some parts of the world malaria is still pandemic.
在世界上一些地区疟疾仍在大规模流行。
n. a pandemic disease
大流行病
例：
a pandemic of influenza
流感大爆发
an influenza pandemic
流感大爆发
To understand what is a pandemic , we have to talk about what an outbreak, an epidemic and a pandemic mean.
为了理解什么是pandemic，我们必须讨论outbreak、epidemic和pandemic的意思是什么。
outbreak
n. a time when something suddenly begins, especially a disease or something elsedangerous or unpleasant
(尤指疾病、危险或不快之事的)发作，暴发，突然发生
例：
an outbreak of cholera/food poisoning/rioting/war
霍乱/食物中毒/骚乱/战争的暴发
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.
当该疾病的病例数突然增加，超过某一特定地区通常预期的病例数时，就会发生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，传染
例：
a flu epidemic
流感的传播
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指向的区域范围更小，而epidemic则指范围更广、人口更多的地区。Epidemic通常限于一个城市或地区，甚至一个国家。
A pandemic is an epidemic that has spread over several countries and continents, and affects a large group of people.
Pandemic是一种已经蔓延到几个国家和大洲，并影响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
