It's probaby way too early, but here is a theory of mine about the Dragon Virus and Goa Magara. I do not know how this came to me, but i just somehow spat out this theory after I woke up the morning of the new year. Yeah... So... i started looking it up for some reason, and stuff started to make sense, so I put it here. It is a bit farfetched and unlikely, but here it is anyway. It comes in 2 parts:
I think the virus has something in relation with the newly introduced class, "chelicera". Now, chelicerata IRL can be a lot of things, spiders for one, scorpions for another one. These both were already introduced to MH (though both of the Akura were classified as carapaceons and those are more like crabs, rather than scorps). These are part of a kind of sub-class of chelicerata, known as arachnids. That gives us the first question: What else could be an arachnid?
How does a virus spread? Mostly, via vectors (organisms, that contain the virus and carry it over from one living organism, to another), for example mosquitos for malaria and fleas for plague an... wait, just stop right here. Plague fleas hide and travel in the fur of rats. And feed on them of course. When rats get in contact with other infectable animals, e.g. humans, fleas jump over and infect humans too. Isn't there an arachnid, that causes infections and live on some animal? Of course there is! Actually there are. Two examples are: mites and ticks. Ticks jump on animals from the tall grass or from an other member of their species or another species. Example: Dog->Human.
How are these apply to Goa Magara?
Goa Magara clearly doesn't die from its Dragon Virus. But the virus doesn't seem to kill its prey either (or only I did not receive any information on its effect, except giving Tigrex an extra load of anger and power.) Either way, lets assume it has some positive effect on Goa Magara, because if there was not, it would not use it. And if it killed it, the virus wasn't used as well. So here's the theory: Except just carrying the parasite (probably a tick of some sort), and having only negative effects it is actually living together with it in a symbiotic relationship. The parasite gets to suck blood from Goa and can safely hide and mate in the fur, while it is applied to other creatures as well. Goa is immune to the virus that the parasite carries and even gets to use it, much like some kind of "venom".
If the mechanics are these, how does the virus work?
Looking at the currently revealed symptoms (anger and power in dire situations) it could increase the production of the hormone, adrenaline. "Why is it helping the user? The prey is getting stronger! It doesn't make any sense!" Actually let us assume, that if the virus is triggered, it will not stop increasing the production of adrenaline in the organism. Main effects of adrenaline: Increases heart rate, increases respiratory rate, increases blood pressure, increases the use of muscles, stimulates glycogenolysis (converting the stuff that is made of unused sugar in the liver back into sugar, and converting sugar to energy), triggers lipolysis (basically converting fat to energy). Some negative effects can be: cardiacal arrythmia, trembling, anxiety, high blood pressure and sometimes pulmonary edema. As I said before, adrenaline is now unstoppable. If the victim doesn't die of exhaustion (having used up every single crisis energy sources) very soon, it will die of either some kind of a cardiacal failure ( most likely a heart attack), or some kind of a hemorrhage, given the 24/7 high blood pressure. Let's not forget the urge to flee (anxiety of some sort). A strong victim ( e.g. Tigrex) will destroy everything in its path to ensure its own safety while hurting itself and probably killing a lot of smaller animals as well. Given the fact that adrenaline doesn't increase any kind of eating, defecating or mating urge, Tigrex will not stop to eat (refill the energy resource), because the urge to be somewhere safe is way bigger.
Goa can feast on a large prey, like Tigrex for weeks, before the carcass is taken over by Jaggi and Velociprey. But lets not forget the creatures it left in its wake while searching for THE spot and exterminating everything that could be a potential threat. Kelbi, smaller Aptonoth, Jaggi, Velociprey are easily carried to its nest, so it can even stock up on meat.
How does it get in contact with Tigrex, one of the most violent monsters of all time?
Probably not easily. If possible, not at all. Easier prey are the packs of Jaggi and Velociprey. They're little-to-no threat to Goa and can carry the infection to their nest. Given the fact that while hunting, and mostly, while fighting for food, situations can become pretty dire (chasing the escaping prey, fighting each other for more meat), the virus can activate, and within a few days, Goa can come back and collect its reward. Tigrex itself gets infected probably while encountering with packs of these minions.
Why doesn't Goa hunt herbivores?
If Goa hunted herbivores, it would not need the virus and made it prime rival of every single other Aptonoth-eater in the game. With this method, Goa is basically controlling the population of the lower-level (and occasionally even the higher-level) carnivores, giving space to the herbivores to reproduce and "inviting" their carnivores back to them, so Goa can hunt again. Based on this, Goa has to have a not too much, but slow enough metabolism, so it doesn't need to hunt too much to annihilate its prey-species, but hunts enough, so its prey won't annihilate the herbivores.
If it uses the virus, why does it grow large and why does it have to climb rocks so easily?
In MH, everything grows large for no apparent reason. Period. Seriously, if it wouldn't grow big enough it could not stand up against its rival species in an occasional fight, or something like that. Climbing rocks so easily? I can't even count the reasons: Finding a nest that no one will find, escaping from battle, escaping after infecting, loosing a pursuer, hiding higher, and the list goes on.
So this is it in a nutshell. It is definitely, as I said before, farfetched as hell, but I felt like it needed a chance.