By that I mean it is a more or less random process where the virus mutates. For one individual the mutations may be benign or silent, whereas for others, the mutations--say in the surface coat of the virus--could result in it escaping the immune system or result in a virus that grows faster.
運氣好的碰到比較良性的病毒，運氣差的就得到破壞力較強或免疫系統較不易辨識的病毒。「運氣」是一般人較容易瞭解的說法，科學上我們稱之為「隨機過程」。