Well, some illnesses may be curable by magic, but others may have been caused by that itself and are not affected by other magics. Therefore herbs/remedies would be required. As we know, viruses and bacteria mutate to fit the world around them, as do the creatures who are afflicted by them. Then there would be some immunity to previously experienced illnesses, which would be matched by newer diseases caused by mutated viruses/bacteria.
Uber scholarly tone there.