Same reason as why it's thirty and not threety in English. More on reddit.com

I'm no expert on the Thirty Years War, but I just finished reading Peter Wilson's book on the war which covers the casualties from the war. Numbers The exact percentage of population decline from the war is difficult to calculate. Many regions of the empire were not affected by the war until Sweden's intervention in 1631, when by that point the population had already grown organically. The most comprehensive survey on the demographic losses from the war was conducted by the 20th-century agrarian historian Günther Franz, who estimated a loss of life as great as 40% in the countryside. However, his work remains controversial and problematic given his membership of the Nazi Party, who frequently manipulated statistics relating to the Thirty Years War for propaganda, but this figure appears to have become a mainstay in popular histories. Other historians have estimated a third of the population was lost, though most modern historians suggest a population decline of around 15-20%. Wilson himself supports a figure suggesting 8 million deaths during the war, including disease. Austria itself was largely spared, suffering only a 2% population decline during the war, though Lower and Upper Austria (hit particularly hard due to uprisings) saw declines of 25% and 17% respectively. Habsburg Italy and Hungary were also hardly affected boasting 13% and 6% population increases respectively. Bohemia suffered the worst with a population loss of 27%. Causes The largest cause of death during the war was disease, exacerbated by troop movement and billeting. Wilson suggests that for every one soldier killed in action, another three died from disease. While typhus was a contributor, the largest cause of death was the bubonic plague. Wilson places military casualties at 450,000 with additional casualties from the Franco-Spanish War that continued another decade past the Thirty Years War. Military personnel killed by disease may be as high as 1.8 million. Violence directed towards civilians caused relatively few deaths, but it did drive a significant amount of civilians from their homes. Emigration was the largest cause of population decline in the regions that saw the greatest loss, but many regions saw this decline offset by immigration. For example, Munich's population declined from 22,000 to 14,000 in between 1618 and 1651, largely due to outbreaks of the plague, but at least 7 thousand settled in the city in that same time period, largely from people fleeing from rural areas. This migration also caused its own problems as it became a large contributor to the spread of disease. As mentioned, troop movement was the largest spreader of disease and large outbreaks can be traced by following armies and refugees. For example, southern Germany was reinfected by the plague when imperial and Swedish troops marched through in 1631. The epidemic then spread to Bavaria when the Swedish troops marched east, and eventually hit Salzburg shortly after likely following fleeing refugees. Those who survived in communities hit by the plague could also be victim to further diseases and malnutrition if military operations disrupted food supply. After an outbreak of the plague in 1633–6 spread by Swedish troop movements, Naumberg recorded another 411 deaths in 1639 largely from malnutrition and another 1,109 between 1641–3 largely from dysentery. Conclusion As mentioned, while military loss of life was high, disease was the largest contributor to the war's population decline. Populations initially grew after the war due to a rising birth rate and drop in mortality. Some travellers called Germany the land of children, with nearly half of the duchy of Württemberg's population being under the age of 15. Unfortunately population growth was further stagnated by subsequent wars in 1672, reversing recovery in some areas. Most modern historians suggest that the empire's population failed to recover to pre-1618 levels until 1710–20. Wilson, Peter H. (2009). Europe's Tragedy: A History of the Thirty Years War. More on reddit.com

Parallelpain

It will depend on where and when. That period saw a major re-thinking of how armies should be supplied. In particular, the continuity of war, and the increasing burden of war relative to the ability of the population to support them, became very bad in the 30YW. But let's back up a bit to the 1500s when the nature of war in Germany (and beyond) to longer, permanent campaigns. What ended up happening was a tendency to introduce enterprisers at all levels, or more critically, financial and services buffers.

Direct contracting for supplies in the 1500s

In the 1500s, state agents tended to directly contract with providers in order to secure supplies. To give a few examples, Henry VIII's royal household in the 1520s would buy supplies from Flemish and French agents in order to get them be ready for Henry VIII's expeditionary forces in France. Spain's Army of Flanders in the 1560s would contract with victuallers to ensure bread and other supplies were made ready for army units marching up the Spanish Road. By the large they were quite successful -- it was said even soldiers appreciated they could always rely on their daily bread.

When armies are on "stand down", they would often be billeted with the population. In the army of Flanders, this was usually done in groups of ten, or for convenience, ten groups of ten. Either the local population would be reimbursed (if they are friendly) or they would be forced to accommodate (if they weren't). Of course, as we know, in many cases food or money or both weren't forthcoming, with the expected results.

Of course, things weren't always so easy, especially when soldiers are on the move or when there was a large concentration of them in a given area. If you look at campaigns of Spinola, his army (and his adversaries) focused on riverine ways as they were key lines of supplies and communication. Woe are days when your riverline lines are cut off.

Increasing weight of wars in the 1600s: the top down view

As I discussed here, the 1600s saw increasing weighing down of war upon Germany. One outcome being, royal households prove unable to keep up with the increasing scale of challenge.

At the lowest level, victuallers were still the key providers. But how they were organized and contracted changed, as the scale changed. For example, the 1507 French campaign in Italy had a supply line stretching 12 miles long. How could they be organized efficiently? The answer was to push the responsibility to the commanders of the armies themselves. This is how figures such as Ambrosio Spinola became important. When he embarked on his enterprise, he personally had zero military experience. His family Spinola, together with the Tassis/Taxis family, were instrumental in setting up the postal and logistics system along the Spanish Road from Milan to Luxembourg and the rest of Southern Netherlands. And his brothers had been involved in various military ventures, although mostly on galleys. So he was perfectly set up for the endeavor. He acted as both banker, supplier, and military contractor to Philip III. One stop shop instead of burdening Philip III's bureaucracy.

Any shortages tended to be exacted from the population itself. In the 80YW commanders would descend upon villages friendly or otherwise, and demand contributions to the war effort. Here too, the right to loot was an important factor, as it allowed for a victorious army to win itself reparations for its shortages, casualties, and suffering.

The culmination of an effort to make all of the above systematic was perhaps Wallenstein himself. Beyond what Spinola did, Wallenstein had set up a "state within state" system whereby he controlled not only the army and its logistics, but also the collection of "revenue" through taxation and other means, to supply his army. In this sense, he was a highly innovative commander. He had come at just the right time, too, as the Imperials needed help in 1625 as Tilly's Catholic League forces were overstretched and Spain's army of Spinola was tied up in the Low Countries, and there were rumors of new movements by Bethlen Gabor from Transylvania. So Ferdinand made Wallenstein "chief of all our troops already serving, whether in HRE or Netherlands," and to "create a field army, whether from existing units or new regiments, to be 24,000 men in all." Now, Ferdinand did not nearly have money to pay for all this, so as the campaign progressed, Wallenstein was rewarded with confiscated estates from HRE princes. In the end, this became Wallenstein's downfall as following Lutzen he had been forced to camp in friendly territory lest Sweden come upon him, and yet his theoretical sovereign wanted to him to camp in enemy territory instead, so that friendly territory would not have to bear the burden of hosting this army. To Wallenstein, this was tantamount to asking him to fight under significant disadvantage, and the rest is history.

Increasing weight of wars in the 1600s: the bottom up view

What I wrote so far may seem simple and brilliant. But to peasants in Germany, it was nothing but. The early 1630s probably saw the worst in terms of looting and rampaging the countrysides, that is still remembered today. But commanders and soldiers weren't ignorant of the challenges. They knew that hungry soldiers tend to steal, and hungry peasants tend to flee, and then all left would be hunger and death. As such, at all levels commanders knew to

... [make] a systematic survey of food stocks in private hands in the city, requisitioning them stage by stage until there was little left.

At this point there was a lot of negotiations done, captains paying off peasants to send his soldiers food, asking peasants for contributions either in money or in kind, and colonels pawning off their assets to keep their regiments intact. Threat of force went a long way, up to and until peasants gave up their dwellings and fields and went elsewhere.

When the Swedes left again on 17 September he summarises his complaints ‘of such bestiis’ who ‘not only filled all the streets with filth and rubbish but also damaged the houses beyond all measure’, and who ate the populace out of house and home as well as stealing whatever they could (Ma.574). His memory seems to have been short, as five months later he complains that friendly troops quartered in Freiburg ‘caused great damage and inconvenience, the like of which no enemy had done before’ (Ma.583). In his briefer account of the later war years Mallinger does not return to this theme, making no complaint during the six-year occupation from 1638 to 1644.

And further speaking on the situation in Brandenberg in 1637,

We were up to our necks with the collection enforcers they set upon us, as numerous as locusts – at times over 40. And we had to give them their rations or subsistence money too, more than 60 taler a day,…not to mention what they pinched and pilfered from people – cattle, sheep, bread, grain, everything from their farms.…The field chaplain and the regimental hangman haven’t come to extort from us yet, but apart from them practically everyone has been here to enforce contributions from Beelitz.

This is where Wallenstein's system was quite brilliant, for he not only convinced the HRE emperor to endow him the authority, he also convinced peasants, village elders, and town councilors that they would be much better off agreeing to his systematic contributions and taxation system rather than Wallenstein push this responsibility off downwards to his colonels and captains.

References

In the English language, it is only now that attention started to be paid to this aspect of war. I would start with Parrott's The Business of War, then check out Mortimer's Eyewitness Accounts of the 30YW and Martines's Furies.

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God damn you Sweden, all the misery in Northern Europe is your fault. This was true 400 years ago and it's true today. More on reddit.com