The trenches of World War I were breeding grounds for an extraordinary range of diseases, many of which killed or disabled more soldiers than enemy fire. Louse-borne infections like trench fever and typhus, waterlogged conditions that destroyed feet, bacterial skin infections, tetanus from contaminated soil, typhoid, dysentery, and eventually the devastating 1918 influenza pandemic all swept through the lines. Some of these illnesses were ancient enemies of armies, while others were new enough that doctors struggled to identify them.
Trench Fever
Trench fever was, in many ways, the signature disease of the First World War. Soldiers developed sudden high fevers that spiked every five days or so, along with severe pain in the shins and back, headaches, and a general misery that could last for weeks. The illness wasn’t usually fatal, but it removed huge numbers of men from the fighting for extended periods. Entire units were crippled by it at once.
At first, nobody knew what caused it. Two commissions in 1918 finally established that body lice transmitted the disease. The bacterium responsible, initially called Rickettsia quintana, was consistently found in the gut and feces of lice that had fed on infected soldiers. Its role was accepted in the 1920s, and the organism was later reclassified as Bartonella quintana after being cultured in the 1960s.1PubMed Central. The centenary of the discovery of trench fever, an emerging infectious disease of World War 1 The disease was essentially inseparable from the lice. Where lice thrived, trench fever followed.
Typhus and the Fight Against Lice
Epidemic typhus was far deadlier than trench fever, and the two diseases shared the same vector: the body louse. Typhus produced severe fevers, rash, delirium, and could kill a substantial fraction of those infected. It had ravaged armies for centuries, and the crowded, unsanitary conditions of the trenches were ideal for its spread.
The link between lice and typhus was a relatively recent discovery at the war’s outset, identified by the French researcher Charles Nicolle just a few years before fighting began. The Serbian army was hit particularly hard by a typhus epidemic in 1914 and 1915. Once Serbian military doctors accepted Nicolle’s findings, they channeled their efforts into delousing on a massive scale. Unable to wait for factory-made autoclaves, they improvised. An old brick factory was converted into a “central disinfection station” with a dry hot-air oven and an attached bathhouse. It could process 1,200 soldiers a day, delousing their clothing in the heated chamber while the men bathed, then returning the clean clothes to them. Without the bathing component, the station could decontaminate 4,000 to 5,000 items of clothing and bedding daily. This improvised approach became one of the most effective methods worldwide for containing typhus for nearly three decades, until DDT became available.2Glasnik javnog zdravlja. Contribution of the Serbian military medical service to global medicine in 1914/15
On the Western Front, the British and French armies also invested heavily in bath-and-delousing stations behind the lines. Soldiers rotated through them when they came out of the trenches, but within days of returning to the front, most were re-infested. The louse problem was never truly solved during the war; it was only managed through relentless effort.
Trench Foot
Trench foot was the painful, sometimes crippling result of standing for days or weeks in cold, waterlogged trenches. Along the roughly 15,000 miles of trenches on the Western Front, the condition caused serious losses among fighting troops, producing swollen limbs, impaired sensory nerves, inflammation, and in severe cases, tissue loss through gangrene.3PubMed Central. Trench foot–a study in military-medical responsiveness in the Great War, 1914-1918 It was a particular problem in the low-lying sectors of Belgium and northern France, where water could not drain and soldiers sometimes stood in mud and water up to their knees for days on end.
Medically, trench foot is classified as a nonfreezing cold injury. Moisture is required to produce the condition. The affected limbs typically pass through four stages during and after exposure: the cold exposure phase, a post-exposure period, a phase of increased blood flow, and then a longer aftermath of lingering symptoms. Most soldiers who developed it reported losing feeling in their feet for at least 30 minutes, followed by pain or abnormal sensation when the feet warmed up.4PubMed Central. Nonfreezing Cold Injury (Trench Foot) The nerve damage could be substantial and long-lasting. Modern research suggests that trench foot operates as a “painful vaso-neuropathy,” where tissue damage from reduced blood flow leads to abnormal growth of small blood vessels and disordered nerve fibers, producing chronic pain.5PubMed Central. Trench Foot or Non-Freezing Cold Injury As a Painful Vaso-Neuropathy: Clinical and Skin Biopsy Assessments
Physicians and military officers tried many approaches before settling on a combination of basic hygiene and enforced discipline. Commanding officers were eventually held responsible for trench foot rates in their units. Soldiers were ordered to change into dry socks regularly, rub whale oil into their feet, and avoid prolonged immersion. These measures reduced but never eliminated the problem. For soldiers who could not avoid prolonged exposure, staying active, keeping well-nourished, and changing socks at least daily remained the most effective preventive measures.4PubMed Central. Nonfreezing Cold Injury (Trench Foot)
Bacterial Skin Infections and Scabies
Skin disease was so common in the trenches that it became one of the leading causes of men reporting sick. Data from the war shows that bacterial infections accounted for roughly 42% of all skin conditions, scabies for about 28.5%, eczema and dermatitis for 13%, and acne for about 5%.6British Journal of Dermatology. The changing face of skin disease in warfare: from World War I to modern times These numbers paint a picture of soldiers perpetually struggling with infected cuts, boils, and abscesses while simultaneously being eaten alive by mites.
Scabies was spread easily in the cramped quarters of dugouts and billets, where men slept shoulder to shoulder in shared blankets. The itch mite burrowed into the skin and caused intense itching, especially at night, which made an already miserable existence worse. Treatment involved sulfur-based ointments and, ideally, clean clothing and bedding, commodities that were difficult to supply at the front. Bacterial infections arose from the combination of minor wounds, insect bites, scratching, and constant contact with filthy water and mud. Without antibiotics, which did not yet exist, infections that would be trivial today could become serious enough to require evacuation.
Tetanus and Soil-Borne Infections
The heavily manured farmland of Flanders and northern France was rich in Clostridium tetani, the bacterium that causes tetanus. Any wound contaminated with soil carried a risk of the disease, and in the opening months of the war, tetanus was a serious threat. Shrapnel and bullet wounds were rarely clean; they drove mud, fabric, and debris deep into tissue, creating ideal conditions for the bacteria to grow.
The military response was one of the war’s genuine medical success stories. Routine prophylactic injections of anti-tetanus serum were given to wounded soldiers removed from the firing line. A steep fall in tetanus cases followed on both sides of the conflict. Anti-tetanus serum prevented life-threatening tetanus among several hundred thousand wounded men, making it one of the most successful preventive interventions in wartime medicine.7PubMed. Prevention of tetanus during the First World War Gas gangrene, caused by related soil bacteria from the Clostridium family, was another feared complication of battlefield wounds. It could destroy tissue rapidly and was often fatal; amputation was sometimes the only option.
Typhoid Fever
Typhoid fever had been the great killer in previous wars, claiming more soldiers than battle in conflicts as recent as the South African war of 1899–1902. By the time WWI broke out, a typhoid vaccine existed, and the British Army made use of it aggressively. A combined vaccine covering typhoid plus paratyphoid A and B (known as the “TAB” vaccine) was introduced for British troops in 1916. During the last three years of the war, over 90% of British soldiers were inoculated.8PubMed Central. Typhoid and the Military in the Early 20th Century Some later doubt was cast on how well the wartime TAB vaccine worked against paratyphoid, but it remained effective against typhoid itself and offered substantial protection.
The result was a dramatic change from past wars. Typhoid, which had previously devastated armies in the field, was kept to relatively low levels among the vaccinated British forces. Dysentery, caused by a different group of bacteria spread through contaminated water and food, was harder to control. It caused severe diarrhea, dehydration, and could be fatal, particularly when medical evacuation was delayed. Water purification in the field, including filtration and chlorination, was implemented by military engineers, but sanitation in forward trenches was always precarious. Latrines overflowed, were destroyed by shelling, and sometimes sat dangerously close to water sources and food preparation areas.
The 1918 Influenza Pandemic
The catastrophe of the 1918 influenza pandemic intersected directly with trench warfare. The crowded conditions of military camps and trenches fostered the spread of the virus, which traveled with personnel from camp to camp and across the Atlantic. At the height of American military involvement in the war, from September through November 1918, influenza and pneumonia sickened between 20% and 40% of U.S. Army and Navy personnel.9PubMed Central. The U.S. military and the influenza pandemic of 1918-1919 The impact on operational readiness was enormous. Entire units were laid low simultaneously, and field hospitals that were already overwhelmed with battle casualties had to absorb waves of desperately ill flu patients.
What made the pandemic so lethal was not the virus alone. Research has shown that the initial influenza infection, while severe, was often self-limiting. The real killer was secondary bacterial pneumonia. Bacteria that normally colonized the airways exploited the damage done by the virus to produce devastating lung infections.10PubMed Central. Deaths from bacterial pneumonia during 1918-19 influenza pandemic Autopsy studies from military fatalities confirm this. Examinations of lung tissue from soldiers who died during the pandemic at U.S. Army training camps revealed compelling evidence of severe acute bacterial pneumonia in virtually all cases.11PubMed Central. Death from 1918 pandemic influenza during the First World War: a perspective from personal and anecdotal evidence A separate autopsy series of 68 cases found that every single one had evidence of bacterial pneumonia, and 94% showed abundant bacteria on tissue staining.12PubMed Central. Autopsy series of 68 cases dying before and during the 1918 influenza pandemic peak
In the trenches, this one-two punch of virus and bacteria was especially dangerous because soldiers were already weakened, sleep-deprived, and breathing air contaminated by mud, decomposition, and chemical residues. The pandemic killed more people worldwide than the war itself, and soldiers in the trenches were among the earliest large-scale populations to be hit.
Chemical Weapons and Lasting Lung Damage
While chemical weapons like chlorine and phosgene gas were primarily battlefield tools rather than diseases, their health consequences blurred the line. Mustard gas, first used in 1917, was particularly insidious because it did not just cause immediate blistering of the skin and eyes; it produced lasting respiratory damage that functioned like a chronic disease for survivors.
Sulfur mustard exposure leads to a condition known as “mustard lung,” characterized by persistent cough, difficulty breathing, and chest tightness. Studies of veterans exposed to mustard gas have found that all symptomatic patients showed significant air trapping in lung imaging and marked increases in residual lung volume. Lung tissue biopsies revealed airway injury in half of cases and organizing pneumonia, a specific pattern of lung inflammation and scarring, in the majority. Inhalation of sulfur mustard can produce clinically significant lung disease that persists many years after exposure.13Pathology – Research and Practice. Mustard lung secrets: Long term clinicopathological study following mustard gas exposure More recent research has shown that mustard lung involves a distinctive profile of oxidative stress, where the balance between damaging and protective molecules in the lungs is disrupted in ways that drive ongoing inflammation and tissue damage.14PubMed. Mustard lung with a unique oxidative stress profile as an independent pulmonary disease
Soldiers who survived gas attacks often spent the rest of their lives with compromised lungs, vulnerable to infections and unable to perform physical labor. For many, the lingering effects of gas exposure were a form of chronic disease that outlasted the war by decades.
Shell Shock and the Effort Syndrome
The trenches produced psychiatric casualties on a scale that military medicine had never encountered. “Shell shock” was the term applied to a wide range of symptoms: tremors, paralysis, blindness, deafness, mutism, nightmares, and inability to function, all without any visible wound. When the condition was first identified, doctors assumed it was caused by physical brain damage from the concussive force of exploding shells. Subsequent clinical study suggested this view was too simplistic, and explanations soon swung back and forth between the purely physical and the psychological. Despite vigorous debate, physicians were unable to identify or confirm clear distinctions between the two.15American Journal of Psychiatry. Shell shock and mild traumatic brain injury: a historical review
A related but distinct condition was called the “effort syndrome” or “soldier’s heart.” Affected men experienced pounding heartbeats, breathlessness, chest pain, and dizziness, particularly during physical exertion. Thomas Lewis, a prominent British physician, studied the condition extensively and argued that it was not a structural heart problem but rather a functional disorder. His work emphasized that too much importance was being assigned to heart murmurs detected during examinations, since many healthy soldiers had innocent murmurs that meant nothing.16JAMA. The Soldier’s Heart and the Effort Syndrome The effort syndrome is now understood as an early description of what would later be called neurocirculatory asthenia and eventually recognized as part of the broader spectrum of trauma-related and anxiety-related physical symptoms.
The military’s handling of shell shock ranged from sympathetic to brutal. Some men were treated with rest, occupational therapy, and gradual rehabilitation. Others were accused of cowardice or malingering. The sheer number of psychological casualties forced both medical and military establishments to acknowledge that the mind, and not just the body, could be broken by war.
Malaria Beyond the Western Front
While the Western Front in France and Belgium gets the most attention, the war’s trench systems extended to other theaters where different diseases dominated. Malaria was a relatively minor issue on the Western Front but became a massive problem in the Mediterranean and Middle Eastern campaigns. As more countries entered the war and fighting spread, malaria caused major epidemics in Macedonia, Palestine, Mesopotamia, and Italy.17PubMed Central. Malaria’s contribution to World War One – the unexpected adversary
The Macedonian front was particularly notorious. British and French forces stationed around Salonika suffered staggering malaria rates, and the disease put far more men out of action than enemy combat did. Quinine was available as a treatment, but supply was inconsistent, soldiers often resisted taking it because of its unpleasant side effects, and preventive measures like draining standing water were difficult to implement in active combat zones. In Mesopotamia (modern Iraq), the heat, insects, and lack of infrastructure combined to produce not only malaria but also sandfly fever and a range of gastrointestinal illnesses that sapped the fighting strength of every army that operated there.
How Disease Shaped the War Itself
The sheer variety of diseases in the trenches reflected the extraordinary combination of factors that produced them: millions of men packed into narrow earthworks, exposed to cold, wet, and filth for months on end, living alongside rats, lice, and flies, eating and drinking in close proximity to latrines and decomposing remains. Each disease exploited a different weakness in this arrangement. Lice carried trench fever and typhus. Standing water destroyed feet. Manured soil infected wounds with tetanus and gas gangrene. Crowding spread influenza and respiratory infections with lethal efficiency. Poor sanitation fueled typhoid and dysentery.
Military medical services were forced to innovate at a pace that peacetime medicine rarely demands. Antityphoid vaccination, antitetanus prophylaxis, organized delousing programs, water chlorination, wound antisepsis, and the first large-scale encounters with psychiatric casualties all advanced rapidly under the pressure of trench conditions. Many of these innovations carried over into civilian medicine after the war. The diseases of the trenches were not just a medical catastrophe; they were a forcing function that reshaped how armies and eventually entire health systems thought about prevention, sanitation, and the health of large populations living in close quarters.