@Shinhyung,
Earnest answers to strange questions. I suspect you're trying to lead me to some point, but you've asked very unusual questions, probably though because I believe you didn't articulate them as you meant to. Not sure if it's deliberate or not, but if your aim is to make a hierarchy of biological systems (i.e. we'll die in 5 minutes without breathing but no virus can kill us that fast), it's a moot point because our biological survival requires in ALL cases gas exchange as well as functional immunity against microorganisms. Which is more important? You can't survive infancy without an immune system. What are the metaphysical implications of this? None.
My former area of research (and I did a project on this in West Africa during my final year of med school) was the interaction of malnutrition with infectious diseases. Long story, but malnutrition syndromes are partially triggered by infections, and conversely susceptibility to infections is worsened by malnutrition. Two of our MAJOR interfaces with the environment around us are 1) the ability to take in and process food, and 2) the ability to recognize and defend ourselves against pathogens (but recognize and tolerate our own body at the same time) -- and these are intimately related physiologic systems in both health and disease states.
Now, let's go back to your questions and see how many ways they can adequately be phrased and answered, which I must do because the questions weren't very clearly articulated:
How long can you live without a viral attack?
If you mean how old must someone be before a viral infection, the answer is some happen even before birth (CMV, herpes, HIV, hep B, rubella), and nearly all will have a viral infection in the first couple months of life.
How long can you live without a viral attack?
If you mean how long can someone live if there is no viral infection, the answer is it depends on all the other things that might kill you instead. To my knowledge we don't depend on any virus to live, but that's probably not 100% true, I'm sure some of our endogenous gut bacteria have commensal phages.
How long can you survive if you cannot fight off viral infections?
In the case of the most complete congenital immune deficiency disease (SCID, or severe combined immunodeficiency), most children will die in the first year of life if they do not receive a bone marrow transplant, often from viruses but often from other microbes.
How long can you live by holding your breath?
"By" holding your breath? It's a false question, because holding your breath doesn't make you live.
How long can you live by holding your breath?
Is this a trick question? Because you MUST hold your breath fleetingly between inhalation/exhalation. Breathing at 10-15 times a minute is necessary for normal exchange of carbon dioxide. If you breath too fast and blow off too much CO2, then you can become alkalotic and this can be life threatening (though ordinarily alkalosis will suppress respiratory drive).
How long can you live by holding your breath?
As I said in my previous post, if your question is really "how long will you SURVIVE by holding your breath", the answer is that you will live a normal life span as long as you're able to pass out somewhere safe. Toddlers have breath holding spells when they have tantrums. They pass out and start breathing again immediately.
How long can you live by holding your breath?
Do you mean how long can you survive without any breathing at all? A few minutes.