How viruses direct healing
To me, viral infections enhance healing processes in the body, and can be ultimately strengthening, but this "targeted healing" comes at a price - the draining of the body of its precious vitality during the process, and the causing of discomfort.
Recovery from the mild illness is like a rebirth, and the body is changed, and perhaps improved, at the end of it.
For those whose vitality is very low, this emergency healing session may be more that they can handle and passing* will be the result. (*Death of the organism and release of the soul.)