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Building on Josh King's answer to this question concerning the feasibility of a disease which dies with its host, can a disease be contagious by airborne means while not being contagious by fluid exchange or touch?

I'm trying to define an apocalypse virus story, but I keep running into scenarios where the zombie/berserker still constitute an unescapable threat even after they've all been cut down by automatic gun fire. Their fluids carry the virus into local water table, their flesh gets eaten by carrion birds. Even if none of that happens, the defeated zombie/berserker corpses will soon develop a variety of dangerous secondary infections unless the surviving uninfected can dispose of those corpses safely.

I'm discovering why most zombie stories don't have happy endings. The aftermath of any zombie outbreak is a continually repeating series of future disease outbreaks.

So to combat that, I am trying to limit the contagious stage of the disease to its victim's living and/or breathing moments. I'm trying to come up with a rational explanation for the death of a disease's host, also marking the end of the disease's threat to its survivors.

So can a disease be only contagious by sharing breaths?

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  • $\begingroup$ Don't just include a link to what you're referencing. Actually put it in the question please. $\endgroup$ – Aify Jul 10 '16 at 22:12
  • $\begingroup$ I've edited the link to more fully describe what the previous question was asking. Please let me know if further clarification is necessary. $\endgroup$ – Henry Taylor Jul 10 '16 at 22:19
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Let's imagine that the organism causing the disease (a virus) has two states, depending on whether or not it's inside its host.

State #1: Active

In this state, the organism is inside the host. It's feeding on nutrients it finds and subsequently reproduces, sending its offspring or replicas throughout the host's body. When it's in this state, it needs nutrients X, Y, and Z to stay actively reproducing; these nutrients can only be found inside the host's living body.

State #2: Dormant

In this state, the organism is outside the host. It can no longer access nutrients X, Y, and Z, and so it can no longer reproduce (this goes along with the definition of a virus). Therefore, it goes into a state similar to hibernation - a low power mode, if you will. For this, it needs nutrient A, which is only found in the ambient atmosphere. If nutrient A is detected and nutrients X, Y, and Z are no longer detected, the organism goes into State #2.


Reason for a lack of contagion after the host's death

Nutrients X, Y, and Z are only found in a living host's body while the host is in the grips of the disease, and Nutrient A is only found in the air. When the host dies, the system breaks down and Nutrients X, Y, and Z are no longer available in the correct form. The organism carrying the disease has no way to store energy, and so it immediately dies.

Additionally, the organism must resist all attempts to exit the host's body, except when it is forcibly exhaled. It will do what it can to remain in the host's internal organs, so it will try to avoid the respiratory system, skin, and areas with fluid where it might be released against its will. However, when it leaves the body encased in fluid, it dies, because none of the four nutrients are available. It also cannot spread via touch; the organism will do whatever it can to avoid the surface of a host so it can remain inside.

These can be explained by simply having the organism be in a much better position to survive if it stays inside the host. For instance, it could only be able to spend a very, very, very short time in State #2 before dying.

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So can a disease be only contagious by sharing breaths?

Not really, but it may not be relevant.

Once the virus or bacterium is inside the host and actively replicating, it can't be diffusing through breathing, it's diffusing through the blood or internal body fluids (e.g. lymph). It stands to reason that a sufficiently massive transfusion of said blood or infected fluid would also propagate the disease.

On the other hand, few people would share blood or sufficient quantities of bodily fluids with an evidently sick person. So for all intents and purposes, if the blood-borne form is not-infective enough, the organism is only contagious through the breath.

So we just require the main transmission vector to be Flügge droplets, and also posit that the disease organism is a bacterium and can't sporulate efficiently, so only "fresh" breath is infective. Entering a room five minutes after an infected person sneezed inside would then be harmless except in the most controlled conditions of temperature and humidity.

A problem would be how to justify the disease existing in the first place - it would need to have "made the jump" from some other host to humans. Possibly we could have some animal act as a host but without any extreme consequences, in order to act as a reservoir for the disease.

But really, can we make even trasfusions harmless?

Yes, just not with organisms that I know of. I think that this would require a novel lifecycle in which the reproduction of the disease organism can not take place in blood or fluids, but only at the pulmonary alveolar interface. The reproduction inside an organism would then happen deep in the lungs only. I imagine that would require to make the disease organism depend on some very specific substance to enter the reproductive stage - for example DPPC - hence the difficulty for such an organism to have evolved on Earth.

On the other hand, for a genetically engineered bioweapon this might seem a good choice. One of the big problems with bioweapons is to ensure they are infective enough to hit the target, and yet not infective enough to wipe out both combatants. Having it reproduce by short-range breath only would seem an option. After all, what could go wrong?.

Surfactant dependency might also mean that you could treat infected air with some shampoos and have good chances of rendering it harmless.

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  • $\begingroup$ +1 Thanks! Interesting idea that it can only reproduce in the lungs. Didn't know that was possible but if it is and is therefore believable, it should be very easy to explain to non technical readers of my story. $\endgroup$ – Henry Taylor Jul 11 '16 at 12:24

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