“The flux” is an old term for dysentery, a severe intestinal infection that causes bloody diarrhea, cramping, and dangerous fluid loss. The name was used from roughly the 1600s through the 1800s, and you’ll most often encounter it in historical accounts of wars, sea voyages, and colonial settlements. It was one of the deadliest diseases of its era, and the condition it describes still kills tens of thousands of people every year.
Where the Name Comes From
The word “flux” comes from the Latin fluxus, meaning “flow,” and it referred to the uncontrollable flow of fluid from the bowels. Eighteenth-century physicians used several variations depending on severity. “Bloody flux” was the most common and described the hallmark symptom: frequent, blood-streaked diarrhea. “Violent flux” and “obstinate flux” described cases that were especially aggressive or refused to resolve. All of these terms map onto what modern medicine calls dysentery.
What Causes It
Dysentery is caused by either bacteria or parasites that infect the lining of the large intestine. The bacterial form, which accounts for most cases worldwide, is triggered by a group of bacteria called Shigella. Four species exist, with Shigella sonnei being the most common in the United States and Shigella dysenteriae causing the most severe illness. Amoebic dysentery, the parasitic form, is caused by a single-celled organism that thrives in contaminated water and is more common in tropical regions.
Both forms spread the same way: through water or food contaminated with fecal matter. This is why the flux tore through ships, army camps, and any place where large groups of people lived in close quarters with poor sanitation. On transatlantic slave ships, it was likely the leading cause of death among captives, who were packed into holds with no clean water and no way to maintain hygiene.
Symptoms and Timeline
The defining symptom is bloody, mucus-filled diarrhea, often accompanied by fever and intense abdominal cramping. Historical physicians described “frequent discharge of blood from the bowels” and excrement “mixed with flimsy sharp matter” that caused “severe gripping pains,” along with vomiting. Symptoms could last over two weeks in serious cases.
In modern clinical terms, the illness typically follows a pattern. It often starts with watery diarrhea and fever lasting three to four days before progressing to bloody, mucoid stools as the infection damages the colon’s lining. Mild cases resolve within a week, but colitis (inflammation of the colon) can persist for one to twelve weeks. About 25% of people experience relapses. Severe cases involve more than five bloody stools per day and signs of systemic illness like high fever and dehydration.
Why It Was So Deadly
Before modern medicine, the flux killed through dehydration and electrolyte loss. Each bout of diarrhea drains water and essential salts from the body, and without intravenous fluids or even basic oral rehydration, patients could deteriorate within days. Eighteenth-century naval and military accounts describe entire units being “exterminated by the bloody flux” during campaigns, sometimes losing more soldiers to dysentery than to combat.
Even today, the disease is far from harmless. The estimated global incidence of shigellosis is 188 million cases per year, with roughly 164,000 resulting in death. Children under five bear the heaviest burden: in 2021, an estimated 81,800 children in that age group died from Shigella infections. In rare cases, the infection triggers a complication called hemolytic uremic syndrome, which destroys red blood cells and damages the kidneys. This can lead to kidney failure requiring dialysis, and it carries a risk of long-term hypertension and neurological problems.
How It Is Treated Today
The cornerstone of treatment is replacing lost fluids and electrolytes. For mild cases, oral rehydration solutions (water mixed with specific amounts of salt and sugar) are often enough. Severe cases require medical intervention with intravenous fluids.
Antibiotics shorten the illness and reduce the risk of spreading it to others. The current standard first-line treatment is a type of antibiotic called a fluoroquinolone. However, antibiotic resistance is a growing concern with Shigella worldwide, and some strains no longer respond to common drugs. When resistance is present, doctors turn to alternative antibiotics, though these tend to be more expensive or harder to administer. Most healthy adults recover fully within a week or two with proper treatment.
Prevention Still Comes Down to Clean Water
The same thing that made the flux so devastating on 18th-century ships makes it dangerous today in disaster zones, refugee camps, and communities without reliable sanitation. Prevention centers on breaking the fecal-oral transmission chain.
- Safe drinking water: Boiling water for at least one minute kills the bacteria. If boiling isn’t possible, adding 8 drops of unscented household bleach to one gallon of clear water (16 drops if cloudy) and waiting 30 minutes also works.
- Hand hygiene: Washing hands with soap and clean water before eating, before preparing food, after using the bathroom, and after changing diapers. Alcohol-based hand sanitizer with at least 60% alcohol is an alternative when soap isn’t available.
- Food safety: Cleaning food preparation surfaces with soap and safe water and allowing them to air dry completely before reuse.
- Sanitation: Keeping human waste away from water sources. Washing clothes and bathing at least 100 feet from any drinking water supply.
The flux may sound like a relic from another century, but the underlying disease remains one of the most common causes of bloody diarrhea worldwide. The difference between then and now is access to clean water, rehydration therapy, and antibiotics. Where those basics are available, dysentery is manageable. Where they aren’t, it kills in much the same way it did 300 years ago.