2. 它由什麼所引起？
本病（的病原體）全稱叫人類免疫缺陷病毒（Human Immunodeficiency Virus，HIV）（為了方便記憶，中國大陸將它簡稱為愛滋病病毒或HIV。除非必要，本節下面都使用這兩個稱謂——譯者注），它侵害和逐漸摧毀人體的免疫系統。所以，隨著時間過去，機體變得容易受到所有類型的“機會感染（opportunistic infections）”侵犯，也就是通常並不損害健康人，但是有機會襲擊免疫力脆弱或缺乏免疫力的機體感染。最終，機體免疫力缺乏到如此的程度，以至於不再有能力對抗外界任何感染，並且導致“免疫缺乏綜合征”，既這種疾病典型而又嚴重的複合症狀。疾病發展到這個階段就稱為獲得性免疫缺陷綜合征（Acquired Immune-Deficiency Syndrome，簡稱AIDS）。此時，患者的病情越來越嚴重，直至最終導致死亡。

1. What is it?
We are talking here about stages of the same disease - the first stages of a viral infection (HIV-infection) and its serious, final stage (AIDS)

2. What causes it?
The disease is caused by the Human Immunodeficiency Virus (HIV) which attacks and gradually destroys the immune system. Over time, therefore, the body becomes vulnerable to all sorts of “opportunistic infections”, i.e. infections which normally do not hurt a healthy person, but which find an opportunity to strike a body when its defenses are weak, i.e. when its immune system is deficient. Eventually, the deficiency becomes so great that the body is no longer able to fight back and develops an “immune deficiency syndrome”, i.e. a combination of severe symptoms typical of this disease. This is the stage called AIDS (Acquired Immune-Deficiency Syndrome). The patient’s health problems become ever more severe and finally lead to death.