Is it possible that someone can have a very serious disease even life threatening yet have no physical symptoms. thus having it go untreated for a length of time and eventually dying?

Not sure...: ...that your question makes sense. We all die from something and some even die in "perfect health." It is very unlikely that one has a serious illness without serious signs and symptoms. I can think of a few exceptions to this rule such as critical stenosis of an artery supplying the heart or brain, but even they frequently provide some warning.

Possible but rare: Occasionally this does happen with certain cancers, but generally speaking life threatening diseases present with physical symptoms early on.

Yes but...: Sounds as if you have a specific concern. The best way to address this is to express you concerns clearly and openly with your doctor. A forum like this will not give the the answer you are looking for and will only cause more questions