No one knows exactly how King Baldwin IV of Jerusalem contracted leprosy, because the bacterium that causes it can spread silently for years before symptoms appear. What historians do know is that the disease was first noticed when Baldwin was nine years old, around 1170, and that leprosy was widespread in the Crusader States during the 12th century. The most likely explanation, based on modern epidemiology, is that he was exposed through prolonged close contact with an infected person sometime in early childhood.
How the Disease Was Discovered
Baldwin’s tutor, the chronicler William of Tyre, left one of the most detailed firsthand accounts of a leprosy diagnosis in medieval history. He described a moment when Baldwin was playing with other noble boys and they began pinching each other on the hands and arms, as children do. The other boys yelped in pain, but Baldwin didn’t react at all. William wrote that the young prince “bore the pain, altogether too patiently, as if he did not feel it.” When this happened repeatedly, it was reported to William, who recognized the numbness as an ominous sign.
A medical examination confirmed what William feared. The right side of Baldwin’s right arm and hand were partially numb. Notably, there were no visible skin lesions or sores at that point. The loss of sensation without outward marks is consistent with the early stage of leprosy, when the bacterium is already attacking nerve fibers but hasn’t yet caused the disfigurement people associate with the disease.
The Most Likely Route of Infection
Leprosy spreads primarily through respiratory droplets. When an infected person coughs or sneezes, they release tiny aerosols containing the bacterium, and prolonged, close contact with that person is the main risk factor. Direct skin-to-skin contact may also play a role, though airborne transmission is considered the dominant pathway. The disease is not highly contagious. Most people who encounter the bacterium never develop symptoms because their immune systems clear it. But for the small percentage who are genetically susceptible, an infection can take root and progress over years.
Baldwin grew up in Jerusalem at a time when leprosy was endemic across the region. The Crusades themselves helped spread the disease between the Middle East and Western Europe, and the Crusader States had a significant population of people living with leprosy. The Order of Saint Lazarus, a knightly order based in Jerusalem, was specifically dedicated to caring for lepers and even counted people with leprosy among its members. In this environment, a young prince interacting with courtiers, servants, soldiers, and religious figures could easily have been in sustained contact with someone carrying the bacterium without anyone knowing it. The incubation period for leprosy ranges from two to ten years or more, so Baldwin was likely infected as a toddler or very young child, well before anyone could have identified the source.
How His Leprosy Progressed
The numbness in Baldwin’s right arm was only the beginning. Over the next 15 years, the disease followed a relentless course. As a teenager, he could still ride horses and fight in battle despite gradually losing sensation in his extremities. At the famous Battle of Montgisard in 1177, when he was just 16, Baldwin rode to the front lines and helped defeat Saladin’s army in one of the most celebrated Crusader victories. He was recovering from malaria at the time, which gives a sense of the overlapping health burdens he carried.
By his early twenties, the damage accelerated. In 1182, he stayed on the battlefield during the Battle of Le Forbelet despite his illness and intense heat. But by 1183, Baldwin could no longer walk without support or use his hands. His eyelids lost the ability to close, which dried out his corneas and left him blind. When he accompanied his army to lift the Siege of Kerak in late 1183, he traveled in a litter slung between two horses, unable to fight but still commanding enough loyalty that his mere presence on the field united the fractious Crusader barons. He appointed Raymond III of Tripoli to handle actual battlefield command.
Baldwin died in March 1185, a few weeks before his 24th birthday. In an era with no effective treatment for leprosy, surviving 15 years from the onset of symptoms while simultaneously ruling a besieged kingdom was remarkable.
Why a King Couldn’t Escape It
Modern readers sometimes wonder how a royal child, presumably surrounded by the best care available, could contract a disease associated with poverty. But leprosy doesn’t discriminate by social class. It is caused by a slow-growing bacterium that spreads through ordinary human proximity, and in the 12th century no one understood how it was transmitted. There was no way to screen the people around the young prince, no concept of quarantining presymptomatic carriers, and no treatment that could have stopped the infection once it took hold.
Medieval physicians tried various remedies for leprosy, including herbal poultices and dietary changes, but none had any real effect on the underlying disease. The best that Baldwin’s doctors could offer was palliative care for his worsening symptoms. The bacterium steadily destroyed the nerves in his limbs, led to secondary infections in his insensate hands and feet, and eventually attacked his facial nerves, causing the blindness that marked his final years. What made Baldwin’s case historically exceptional was not the disease itself, which afflicted thousands across the Crusader States, but the fact that he refused to step aside and continued governing through physical deterioration that would have incapacitated most people far sooner.