I have conceptualized a disease that rapidly kills cells in the human body and replaces them with modified copies over a period of 7-10 days. After this time, every cell in the infected person's body should be replaced with altered equivalents that confer biological immortality. Since the "disease" is actually caused by a certain entity's biology-warping powers, the immune system is unable to protect against it.
My question is this: What would actually happen to somebody whose cells were rapidly dying and being replaced like this? I'm afraid that the results might be catastrophic, and I'm not sure it would even be possible to survive the process. My current most dire estimate is that the symptoms might resemble body-wide necrosis in every organ, or extensive radiation burns throughout the body's tissues.
I am hoping that the reality is not so severe as this, since I can at most afford for the external symptoms to be about as severe as a mid-level sunburn (and preferably not detectable at all). But if that isn't realistic, I may have to rethink the timeframe of the infection or invent an alternative.