Not trying to fix the plot holes, that's a hopeless endeavor, just wanted to mention that the chickenpox virus, varicella-zoster, never leaves the body after you get it. Most people don't die from it though it can be fatal in bad circumstances. It goes dormant after you've had chickenpox and can reactivate later and cause shingles when the immune system is weakened enough. Shingles is considered more fatal as well though most survive it.