Compared to how tough most infectious disease germs are, the AIDS virus--the "Black Death of the 20th century," as it has been called--is extremely delicate. It dies virtually as soon as it leaves the human body. That is why it can only be transmitted by intimate contact such as sex.
比起一般傳染病病原的頑強剽悍，號稱「世紀黑死病」的愛滋病毒其實非常嬌嫩，幾乎一離開人體就會死亡，所以只能藉著像性行為這樣親密的接觸傳遞。