One way to picture it: something that does not stay inside one body — it crosses the air between people and takes hold in the next person. The key is not that it is bad, but that it moves from person to person on its own.
From the word’s history
From Latin inficere 'to dye, stain, taint' (in- 'into' + facere 'to do, put'). The old image is of something being put into a substance and colouring it through — which is what a spreading illness, or a spreading mood, does to the next person.
字根來自拉丁文 inficere「染上、沾染」(in- 進去 + facere 做、放)。原本的畫面是把某種東西放進去，讓整塊材料都染了色——傳染病和會擴散的情緒對下一個人做的，正是這件事：不是碰一下，是整個染上。
Wiktionary: infectious < New Latin īnfectiōsus, from Latin inficere (in- + facere)
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原始詞典資料4 筆記錄
以下是外部詞典記錄的用法，摘錄並精簡格式，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
infectiousadj/ɪnˈfɛk.ʃəs/
- 1
medicinepathologysciencesCaused by an agent that enters the host's body (such as a bacterium, virus, parasite, or prion); often, also, transmitted among hosts via any of various routes (for example, contact, droplet-borne, airborne, waterborne, foodborne, fomite-borne, or bloodborne).
- Cancer is usually not infectious.
- More infectious diseases like the flu are usually less potent.
- 2
medicinepathologysciencesAble to infect others; capable of transmitting pathogens.
- Despite feeling better, the patient is still infectious.
- I doubt that [saliva] is very infectious ... I don't think this is a normal route of transmission.
- 3
Spreading quickly from one person to another.
- infectious smile
- Her enthusiasm for work can be really infectious.
- 4
informalMemorable and invoking excitement or interest.
- Pop music is more infectious than elevator music.
摘錄自 英文維基詞典 English Wiktionary，已精簡格式；依 CC BY-SA 4.0 授權使用，改動後的內容同樣以此授權提供。