First of all, from what I have seen, both دوي/dwy & مرض/mrḍ originally had to do with "illness, being sick" in Semitic. In Arabic, مرض/mrḍ still maintains this as the primary sense, whereas دوي/dwy maintains it in the (rare?) Form I verb دَوِيَ dawiya "to become sick" and the noun دَوًى dawan "illness, disease". However, in Form II of the مرض/mrḍ root (i.e. مَرَّضَ marraḍa), the verb means both (1) "to make someone sick, ill" as well as (2) "to nurse someone; to treat an illness", while دوي/dwy has become more associated with "treating disease" (more specifically, contrary to Form I of the root, the root's Form III (دَاوَى dāwā) and VI (تَدَاوَى tadāwā) verbs and their derivatives have to do with "treating disease" ~ they don't even maintain a double meaning like Form II of the مرض/mrḍ root that alludes to the original sense of "to be ill"). These senses seem opposite to each other and I am curious to know how this happens (especially in the case of Form II of the مرض/mrḍ root, where both opposing meanings are not just contained in the same root, but in the same word)?