Great Plague Containment

So, here I will be drawing from the Plague of Justinian. This was a Bubonic plague pandemic affecting mainly the Byzantine Empire. Estimates suggest that deaths range between 13% and 26% of the world population at the time. This was worse than the more famous Black Death affecting the entire continent of Europe 600 years later. I figure the Justinian pandemic would be a better model to use than the Black Death because it is limited to the Byzantines. Which is important to me as the Middle Earth version was meant to attack just one country and the Byzantine Empire is again Gondor's inspiration.
Also important is the context here. Yes, the Black Death killed more both in terms of percentage of total world population and in absolute numbers. But the Plague of Justinian was not that far behind in terms of percentages of the world population killed. And the upper limits of it were not that much lower than the lower limits of the Black Death. The Black Death meanwhile had a much larger population to work with and ravaged the entire continent. So the surprisingly close levels of devastation and significantly higher levels of concentration suggest much greater impact on the affected society.
"The Plague of Justinian". History Magazine. 11 (1): 9–12. 2009.
Rosen, William (2007), Justinian's Flea: Plague, Empire, and the Birth of Europe. Viking Adult; pg 3; ISBN 978-0-670-03855-8.
Which is important for our purposes. Remember the Great Plague getting reintroduced in the Siege of Minas Tirith? Well, that did a whole lot of nothing in terms of weakening the defenses. But viruses do not go away even if the orcs do. So as Gondor begins the process of rebuilding, disease would begin to spread. This plague was originally created by Sauron to wipe out humans for his own political ends. Again, in the case of Minas Tirith, it was introduced specifically as biological warfare to target one specific enemy. So the fact that the Great Plague was both highly devastating and concentrated on one specific society is no accident but actually the plan. This was what Sauron explicitly wanted to happen.
An important note on the biology here. So, in the article on the Great Plague I suggest it is a more deadly than average version of the flu. One whose real world counterpart actually killed people by triggering a hyperactive immune response. So those that had weaker immune systems saw their immune systems strengthened to the point they could successfully fight off the illness. But the healthy adults with otherwise ideal immune systems saw their defenses become so active the immune system turned on the person. Which means the main demographic killed was the same population from which soldiers are drawn. The same people that Sauron was trying to use the Great Plague to kill.

The Conflict

Prelude

So this tactic was well conceived. As evil as we would expect someone like Sauron to think up, but smart none the less. It just happened to take too long to set in for it to be useful. He was defeated before losses could mount enough to gain anything from it. Had Sauron had enough time, he could have just waited. In fact, if he wasn't forced to act prematurely, he could have just introduced the plague and conducted a typical siege. But as noted above, the fact the being who introduced the virus and wanted to exploit it cannot be expected to control it. His removal from the scene before he can exploit it does not mean that the virus stops spreading. It just means that there is no hostile army marching into the opening. Gondor is still screwed.

Deployment

Troops would be the first major population hit. The plague would obviously have started in the Siege of Minas Tirith - where primarily soldiers would be left. But a couple of things are important about these troops. First is a large number came from the Southern Fiefs. Next, they would come into contact with the refugee civilians that would be able to return to the area. Lastly, veterans from this battle would be sent out to conquer Harad, Rhun and Mordor. All have substantial human populations.
Important is the role of Rivendell. Elrond is know for his incredible healing powers. But the problem here is his Ring of Power. Yes, Elrond himself is an impressive healer. But his skills were magically enhanced. So as good as he and the elves generally are on their own, their powers were reduced. He would get involved. Probably personally and with a cadre of other healers. But their interventions would be limited by loss of the One Ring. Then, as noted in the discussion of Cair Andros, their most famous food item is incredibly easy to transport and can be stretched a long time. So they are well placed to provide humanitarian assistance.
And due to the fact the elves already have soldiers in Gondor as military advisers and trainers, the seeds are already in place. Plus, Aragorn is a direct descendant of Elrond's brother. Arwen is his daughter and the granddaughter of Galadriel. Who I might add chose to become mortal in order to marry Aragorn. Which means that Elrond and Galadriel would want to intervene to save their now vulnerable family member.
Lastly, lets remember their foreign policy. The use of soft power and goodwill is how they get by without reliance on military force. Their personal connections will ensure the loyalty of Aragorn. There is no question there. But that loyalty would do no good if Gondor is left broken by disease. Or Aragorn/Arwen die. Meanwhile, providing this aid would secure Aragorn on the throne. Society would be grateful for being saved from this. Most importantly, they will see the elves as being vital to the stopping of the disease and Aragorn as important in getting that aid.
In the worst case scenario, the people who replace Aragorn would still have goodwill towards the elves. This would add an extra layer on top of centuries of education for the elites and cultural exports. Which suggests that to me, there is absolutely no way the elves would not provide aid.
The dwarves would be providing all they can in terms of aid. But their assistance would be geared towards overcoming budget deficits, funding infrastructure and reconstruction projects and economic development. Which are all essential. But not exactly the best for short term avoiding famine or constraining the spread of a pandemic. The elves would be able to fill this role.

Conditions

So, the Great Plague biologically appears to be more akin to the Spanish Flu than the Yersinia pestis that caused the Black Death. Flu would thrive in the conditions after the war. Whenever food production is disrupted on the scale I think the War of the Ring would cause, average per capita food intake goes down. Which hampers the ability to fight illness.
Then, flu is spread in conditions where people are in close proximity to each other. Like in refugee camps. Or military bases. Ship and cities are also good breeding grounds. Due to the policies I propose that Aragorn will take and the impacts of the War of the Ring, there would be an increase in the percentage of people in these conditions. Which then makes matters worse. Sudden increases of population in areas usually means that the heath care systems would be taxed and overcrowding much more likely. Sanitation goes down. War also causes increases in diseases normally too. So this would prove to be a real challenge.

The Engagement

So, lets look at the likely impacts of the disease for a second here. The Spanish Flu infected 30% of the US workforce and killed 2.5% of the workers (0.8% of the whole population). Estimates that that many sick people and an average illness lasting 3 weeks would shrink GDP by 5%. Which means an outbreak of flu - if this is interpreted strictly - would kill 1/12 of the people it infects and would shrink the GDP by 1% per every 6% of the population infected.
Estimates of the Plague of Justinian suggest that 40% of Constantinople's inhabitants were killed and up to 25% of the Empire as a whole. So, Minas Tirith and Osgiliath had pre war populations of 81,060‬. Which means 32,424‬ infected, between 648 and 2,026 dead and economic activity would drop by 6.7%. For the whole of Gondor, there was 1,351,000 people. Which translates to 337,750 infected, killed between 10,808 and 33,775 and shrunk the economy by 4.17%. This would add a whole new layer of misery on the population.
But it is not limited to Gondor. Troops would be moving into Harad, Nurn and Rhun. These slave cultures would see a the same crowded living conditions, poor rations and sanitation known in Gondor as the result of war. The causes of these conditions would be the general lack of concern for the well being of slaves typical in societies that practice slavery. However, the impacts would be the same. The slaves would be hit hard. Rhun would be worse though due to the civil war and foreign interventions there. Plus Mordor and Harad would be incorporated into Gondor and thus benefit from the elven humanitarian aid.
But not for Rhun. Conflicts tend to make it much harder to get needed medical aid into areas experiencing a pandemic. The Ebola outbreak in the DRC is a good example of this. Even without conflict, marginalized groups tend to suffer more from pandemics due to the nature of that marginalization. Nonwhite groups in the US suffered higher rates of mortality in the Spanish Flu than whites.
The impacts of the Justinian Plague demonstrate the impacts that such a plague could have in the ancient world. The Byzantine Empire was struggling to fund many important projects, like the Hagia Sofia. Plus there were the wars that drew a lot of money out of the treasury. This is similar to Gondor. Gondor both has the reconstruction costs and the wars to finance. The fact that reconstruction is more justifiable than a big church is irrelevant. The church was "less important", but also less expensive. The hit to the budget is what is really important for our calculations.
Also important is the human costs of this. Constantinople was losing by some estimates (which many doubt) between 5 and 10 thousand people a day. The city was said to smell of death. Proper burials became practically impossible. Rites were often not performed and bodies were left stacked up in the open. This seems entirely possible to me if Gondor is hit as hard as I suspect here. Such a high toll makes sense. The mortality from flu in American cities was 3-5 times as much in 1918 compared to non pandemic years.
This has more indirect negative impacts on a society on top of this already extreme level of human misery. This disrupted trade and agriculture. Which means that due to the impacts of the War, famine becomes increasingly likely. This and the trade breakdowns provoked increases in food prices for Constantinople. For Gondor, I would expect the pressure to be severe enough that attempts at rationing might be implemented. The strain on the budget would increase too, as the government subsidizes grain in the capital. Bread riots would become increasingly likely. Desperate measures would be needed to ensure mass starvation would not break out.
Anecdotal evidence suggests huge economic costs for the Spanish Flu. In some areas businesses claimed to lose between 30% and 70% of commercial business. The Memphis Street Railway saw 124 out of their 400 employees out at one point. Some coal mines saw 50% declines in production because labor shortages. Some mines came close to closing entirely.
Basically the sudden deaths of large numbers of people meant that there were a lot of estates that needed to be divided. As this was so quick, many did not have wills prepared in advance. The Byzantines needed to pass special legislation to deal with the increase in lawsuits this caused. Gondor would be in a similar position.
The desperation of Justinian was so severe that he demanded the full tax owed by communities be paid. He made no allowances for the decline in economic activity the communities suffered. He also demanded the taxes be not just by the survivors but their dead neighbors too. Which then sounds like a nice recipe for peasant revolts too. Especially when we realize that desperate measures by governments in these situations also tend to involve forced grain requisitions.
This is where the elves and dwarves come in. The budget aid from the dwarves would allow for some moderation of the extraction of taxes. The pressure on farming communities would be lessened due to the elves. The two would allow Aragorn the policy tools needed to appear to be taking action. It must be noted that a lot of the modern means of controlling the spread of disease would not be known. But the impacts due to starvation, poor medical care and economic breakdowns would all be corrected by the foreign aid. Which would contain the disease. Most likely it would still rage wildly. But the worst possible impacts would be stopped.
Just as important is Aragorn looking like he is doing something. This is key. Rulers who are quietly implementing effective policies behind the scenes will still suffer from popular backlash. Not realizing the extent of the help their rulers are providing, the people will condemn their rulers for abandoning them. But ineffective measures loudly proclaimed can calm nerves and help keep people from taking rash actions. Because the elves are seen on the streets passing out food and caring for the starving and sick, people will see the government as effective. Dwarven money will also allow for compensation for losses, building hospitals, continuing food subsidies and so forth. Which the people will see happening.

Outcome

So one of the real life consequences of the Justinian Plague was a war with the Goths. The plague led to a weakening of the Byzantine army to such a degree that the force left in Italy was rather pathetic. The Goths smelled an opportunity to strike. Similar plague induced defeats occurred against the Arabs too.
We also have to remember that the Easterlings would be hit after Gondor. Which means that it would most likely be them that sees the chance to strike. They would most likely launch that invasion I mentioned in the section on their struggles. But this would then risk being what spreads the plague to the Rhun. Which then would destroy their society even more. Another defeat and the triggering of another invasion of their territory. The plague would cause one more trouble the Blue Wizard would have to deal with. Recovery would be even harder and forming a government able to lead that recovery would become less likely.
But just as importantly is the timing. Yes, the main initial battle would be lost by the Easterlings. But the impacts of the plague on both societies would likely weaken both to the point conflict becomes too hard. So the plague would set the conditions for both sides wanting to continue an all or nothing battle to the end. But it would also keep them from following through. So it might be what produces a moderate victory. One where Gondor is the winner but unable to completely impose its will. But a weak and sick territory one needs to rebuild would not be much of a prize. Even if Gondor was able to occupy the whole area.

Aftermath

Remember the lost productivity due to illness? Well, roughly 2.5% of the workers in the US lost their lives to the Spanish Flu. The result was actually higher wages. The classical supply and demand equation appears to have worked. Essentially, fewer workers meant higher wages for those that remained. This is confirmed by the fact the effect was actually stronger in areas where the pandemic was worse. Which tended to be cities.
This means that manufacturing jobs would see a sharp rise in wages. First, there is there a new demand for labor in the factories due to the increasing industrialization of Gondor. Then, the existing workers would be killed dis-proportionally more by the plague. There would have been a bit of a labor shortage anyway as growth in the industrial sector expands faster than the population of the cities. Which is compounded by the actual drop in population. So wages would go up. This then would pull serfs in from the Southern Fiefs and from the former slave territories.
But this would not last. Studies suggest that the babies from mothers who were infected during pregancy saw a 15% decline in the odds of graduating school and a 5%-9% decrease in wages. Apparently the impacts were great enough to lead to life long disabilities and risks for health problems like stroke. Then or course there is the other main impact on families - breadwinners. Should a worker die, their wives and dependent children are left without income.
So the economic impacts would be positive for the newly industrial workers who survive or move in later. But there would be worse economic impacts on the survivors of the victims. Their children will continue to suffer years later. Some businesses will also fold under this pressure, causing unemployment in others. Though it remains unclear how negative this would be. Some would be able to take the new and higher paid factory jobs. Also, new businesses will spring up later based on the new consumer demand. Plus the influx of people seeking those better wages will eventually see wages return to normal. So economically, this would be a mixed bag.