26 all bird species are thought to be susceptible to infection, domestic poultry flocks are especially vulnerable to infections that can rapidly reach epidemic proportions.
答案：(B) While
While 在此是「雖然、儘管」的讓步用法：雖然所有鳥類都可能受感染，家禽群卻特別容易爆發大規模疫情；Not、Since（因為）、Unless（除非）都不合。
while 你先學到的一定是「當…的時候」（While I was cooking, the phone rang.），所以看到句首的 While 就往時間去想，讀不出轉折。它還有一個同樣常考的意思：「雖然、儘管」，功能等同 although。
先看句子的結構。空格後面是 all bird species are thought to be susceptible to infection，有主詞有動詞，是完整子句；逗號後面 domestic poultry flocks are especially vulnerable… 又是一個完整子句。兩個完整子句要接在一起，中間必須有從屬連接詞。這一步就能刷掉 Not——它是副詞，接不了子句，放在句首也不會構成「並非所有鳥類都…」那種說法（那要寫成 Not all bird species are…，而本句逗號後另有主要子句，結構上容不下）。剩下三個都是從屬連接詞，改看邏輯。前半說「所有鳥類都可能受感染」，後半說「家禽群特別容易爆發疫情」——一個是普遍情況、一個是特例，兩者是對比不是因果。句中的 especially（特別是）也在標示「從普遍中拉出特例」，正是讓步的形狀。驗算法：把空格換成 although 唸一遍，意思不變，就是讓步用法。while 的三個意思一起記：「當…的時候」（時間）、「雖然」（讓步）、「而、然而」（對比並列）：Some people like coffee, while others prefer tea.（有些人愛咖啡，有些人則偏好茶。）分辨的方法是看前後兩個子句的關係——同時發生是時間，互相牴觸是讓步，平行對照是對比。
Not 如前所述是副詞，結構上就進不來。Since 當連接詞是「因為」，會讀成「因為所有鳥類都可能受感染，所以家禽特別容易爆發疫情」——但「所有鳥類都會被感染」推不出「家禽特別嚴重」，中間沒有因果。Unless 是「除非」，等於 if…not，會讀成「除非所有鳥類都可能受感染，否則家禽特別容易爆發疫情」，條件與結果的關係講不通。
While…, …（雖然，讓步）、although／though（雖然）、whereas（而、然而）、be susceptible to（易受…影響）、be vulnerable to（易受…傷害）、reach epidemic proportions（達到流行病的規模）。
Avian influenza, or "bird flu," is a contagious disease caused by viruses that normally infect only birds and, less commonly, pigs. 26 all bird species are thought to be susceptible to infection, domestic poultry flocks are especially vulnerable to infections that can rapidly reach epidemic proportions.
Outbreaks of avian influenza 27 be devastating for the poultry industry and for farmers. For example, an outbreak of avian influenza in the USA in 1983–1984 resulted in the destruction of more than 17 million birds 28 nearly US$ 65 million. Economic consequences are often most serious in developing countries 29 poultry raising is an important source of income, and of food, for impoverished rural farmers and their families.
When outbreaks become widespread within a country, control can be extremely difficult. 30, government authorities usually undertake aggressive emergency control measures as soon as an outbreak is detected.