What does the Death Note consider as "original lifespan"? I was reading this question and it got me thinking about what a person's "original lifespan" really was.

Does the original lifespan mean the amount of time a person has from birth to death, ultimately caused by old age (e.g. the longest a person could live barring abnormal health problems or unusual circumstances (e.g. murders, accidents))? 

What if the person contracts a lethal disease? Surely it would shorten their lifespan, but would it affect their original lifespan?

So, take for example:
John has a lifespan of 93 years (as seen through Shinigami eyes). He works for the CIA, though, and at one point comes into contact with a high dose of radiation from a nuclear device. Doctors tell him he only has a couple days to live.

In this case, John's lifespan has been shortened from 93 years to 2 days. If he writes his own name in the Death Note, set to die from a different disease that takes years and years to develop, would he have effectively beaten radiation poisoning? 

Note that the 23-day rule (XVII) would not apply here thanks to rule XVIII.
The original lifespan is the time you are supposed to have alive at the moment of birth, unless someone, Shinigami or human, use their Death Note on you, or you make a deal with a Shinigami shortening your lifespan, or a Shinigami sacrifices themself to allow you to live incrementing your lifespan.


  What if the person contracts a lethal disease?


Once you spent your lifespan you will die, the reasons of death doesn't matter. If you contract a disease you will live until your lifespan allows you to live.

To illustrate this, Amane Misa was supposed to die when the fan attacked her. Gelus knew her life will end, and decided to intervene. Since her lifespan was spent and she didn't die when she was supposed to, Gelus dies transferring his lifespan to Misa, hence she no longer lives with her "original lifespan" but the shinigami's.


  would he have effectively beaten radiation poisoning? 


He could be died because a indirect effect of the use of the Death Note, which caused him to be affected of radiation poisoning, as can be seen in this other answer, but if there isn't Death Note involved, no matter what he should die when their lifespan exhaust, so he could just get lucky and not being poisoned or something happened before hand that prevented him from being poisoned.