Nature does not care which side is ‘right’ in a human conflict. Biology respects no treaties or neutrality, and does not care about the leading articles of our manifestos. In the vast sweep of history, more soldiers have died of disease than enemy action, and the microorganisms have never discriminated according to our human rules, attacking all sides without regard to ethnic or sectarian difference. Blood is blood, and flesh is flesh.

Until 1914, there have been very few defenses against these microscopic opponents. Sixteen years before the first virus is identified, and more than two decades before the first production of stable penicillin, it hardly seems like a fair fight. Without our human focus on such trivial concerns as which flag flies over a particular camp, the Great War can be understood as a historic struggle between the modern conveniences of sanitation and disinfection on the one hand, and the ancient, tiny destroyers of armies on the other.

Typhus arrived in the Balkans with the wars of 1912 and 1913, when the Serbian Army crossed Albania. It then spread to the Austrian lands from which it now returns. In recent weeks, Serbian doctors have noted a surge of typhus cases among the captured Austrians as well as the hundreds of thousands of refugee Serbs who are creating a humanitarian crisis in the country. A Serbian victory this month over the third Austrian invasion of the year results in the capture of many thousands more sick prisoners at the field hospital in Valjevo, where today the Czech doctors have run out of room for the dead; with the fighting, not enough healthy men are available to dig graves.

Fewer than four hundred physicians lived in Serbia when the war started, and they are already all in uniform right now. Between now and the height of the typhus epidemic in May, more than 120 of them will die of the disease while attending the overwhelming tide of patients. One in six Serbs will contract typhus; 200,000 will die of the disease. More than seventy thousand Serbian fighters — nearly half the nation’s small force at the outbreak of war — will also die of typhus.

Typhus is not a new disease in 1914. Formerly known as ‘jail fever’ or ‘ship fever,’ the illness is known worldwide, producing characteristic dark spots only after a twelve-day incubation period that allows the afflicted person to unknowingly spread it to other people before their symptoms become visible. The disease also has a nasty habit of relapsing, making it a primary suspect for the fever that ravaged ancient Athens during the Peloponnesian Wars of the 4th Century BCE. In fact, typhus decimated Napoleon’s invasion force before ‘General Winter’ ran him out of Russia.

For our purposes, the key moment in the life-cycle of the Rickettsia prowazecki bacterium happens when the body louse Pediculus humanus bites its host, invoking one of our most powerful psychological impulses: we scratch, rubbing the creature’s bacteria-filled feces into our damaged skin. Once infected, a human host spreads the bacteria to any louse who bites them. The critters hide in cuffs, hems, inseams — anywhere in our clothing that they can cling out of sight. Infestations become agony, killing the morale of afflicted units. In a very real way, Austria’s Serbian failure in 1914 is due to the louse rather than enemy action. Ejected, the Hapsburgs do not return while Serbia is burning with fever.

When a sickened person dies, the arthropods flee the cooling corpse, jumping on those who tend the patient or carry the body away for burial. If the victim’s companions need a warm coat, they willingly don the victim’s infected garment. Men sleeping in the open will lie close together for warmth, allowing the lice to crawl between hosts all night long. Typhus is a a disease of filth and close conditions, and nowhere in Europe is more filthy right now than the Balkans.

For most people, a hot water bath is only a dream; soldiers and refugees are forced to wear the same clothes every day for weeks on end; basic hygiene is impossible without soap, while medicine and disinfectants are in short supply amd sanitation is rudimentary. A majority of the people in the region — whether soldier or civilian — are living shoulder-to-shoulder, often in the outdoors.

The epidemic soon spreads along the Eastern Front as well. The German army responds to the threat by instituting strict delousing rules, especially later in the war when troops are transferred from East to West. Although the trenches of the Western Front have more than their share of ‘cooties’ spreading the milder, related disease known as ‘trench fever,’ typhus never makes an appearance in the muck and mire of France; if it did, then the war might end much differently.

However, typhus does appear in German POW camps in 1915; governed by regional commands, they are uneven in terms of treatment, facilities, and standards. Thousands of Russian, French, Belgian, and British prisoners die before international attention brings reform.

These are just the opening acts of the pandemic. By 1914, typhus has invaded Russia — a country that is also being torn apart by displacement, famine, and revolution — from three directions, sickening tens of millions and killing as many as half of them. The death toll even causes Lenin to remark: “Either socialism will defeat the louse, or the louse will defeat socialism.”

Prophylactic delousing is a defining feature of the Great War. While picking each others’ nits on a daily basis, British soldiers call the tiny parasites ‘cooties.’ In 1917, Americans sent to the hospital are stripped, doused in ‘cootie oils,’ and their uniforms steamed to kill lice and eggs; they are using a process that has been perfected by the allies through trial and error. By the end of the war, troops on the Eastern Front are already receiving the earliest vaccinations (‘cootie shots’) against typhus, while fumigants and chemical insecticides have proliferated just like any other kind of weapons — except that these are aimed at the tiniest of foes.