He definitely had it at a very young age, but it’s impossible to say exactly how or when he contracted it. We do know how it was discovered though, and how he may have been treated medically and socially, and we know a lot about how leprosy was dealt with in general in the crusader states.

First of all there is the famous description of how his leprosy was discovered. He was being tutored by the historian and archbishop William of Tyre, who recorded:

“It happened that, as he was playing with some boys of noble birth who were with him and they were pinching each other on the arms and hands with their nails, as children often do when playing together, the others cried out when they were hurt, whereas he bore it all with great patience, like one who is used to pain, although his friends did not spare him in any way…finally I came to realise that half of his right arm and hand was dead, so that he could not feel the pinchings at all, or even feel if he was bitten…His father was told, and after the doctors had been consulted, careful attempts were made to help him with poultices, ointments and even charms, but all in vain…It grieves me greatly to say this, but when he became an adolescent he was seen to be suffering from leprosy to a dangerous degree.” (William of Tyre, quoted in Hamilton, pp. 27-28)

So at first he probably just had numbness in his limbs and no one was quite sure that it was really leprosy, and as William says, they tried various treatments. They must have hoped it was some other skin disease when Baldwin was a child, but by the time he was a teenager his symptoms couldn’t have been anything else. Some people advised him to step down as king and go into seclusion like other lepers would, but he refused. Over the years he lost the use of his hands and feet, he was covered with sores, and he went blind. But everyone noted that his mind worked perfectly, and he was fully involved in political and military decisions, even defeating Saladin in battle on a few occasions.

Unfortunately outsiders did not think very highly of having a leper as a king. In the Middle Ages it was considered to be the same disease as leprosy in the Bible. It might not have really been the same, but based on Biblical instructions, lepers were supposed to be excluded from society. They thought leprosy was sexually transmitted, or that it could be transmitted by any contact at all no matter how brief, or it was an outward sign of sin, etc…basically no one knew what it was or how to treat it. So lepers were typically segregated from society, as was suggested for Baldwin as well.

The Muslim pilgrim Ibn Jubayr, who visited Jerusalem in the 1180s, referred to him as a pig, and noted that Baldwin didn’t go out in public anymore. At the same time, the kingdom was threatened by Saladin and ambassadors were sent to seek help in Europe. Western leaders were reluctant to send money and soldiers to help a leper, who they believed must have been a physical symbol of a decadent, oriental kingdom. Baldwin survived until 1185 when he was 23, and Saladin captured Jerusalem a couple of years later in 1187.

As you mentioned, it’s actually a bacterial infection. It can be spread through water or by very close, prolonged contact, but they had no idea about any of that. There are various kinds of leprosy, such as lepromatous and tuberculoid, and also polyneuritic - Baldwin probably had the polyneuritic form at first, which explains the numbness, but it developed into lepromatous when he got older, which explains the sores and blindness and eventual early death.

Hamilton’s biography of Baldwin has an extremely useful appendix by medical historian Piers Mitchell, who examines Baldwin’s symptoms.

“…someone who spent plenty of time with the young prince must have had the disease as he clearly caught it from somebody.” (Mitchell, in Hamilton, pg. 249).

Maybe it was a wet nurse, maybe a family member, maybe one of the other aristocratic kids he was playing with...but it could have been anyone really, it’s impossible to say.

“The general approach to the treatment of those with leprosy complex disease in the crusader period was by modification of diet, bathing in hot springs, the use of drugs, bloodletting, avoidance of sexual activity and segregation in leprosaria.” (Mitchell, in Hamilton, pg. 254)

Baldwin probably went through all of those treatments, although we don’t know anything specific aside from what William of Tyre mentioned, and his apparent seclusion, as noted by Ibn Jubayr.

There were enough lepers in the east that the crusaders eventually learned not to be as terrified of them as people were back in Europe. There was even a branch of the Hospitaller military order specifically for knights who had leprosy, the Order of St. Lazarus (named after one of the Lazaruses in the Bible who was supposed to have had leprosy, although it’s actually not clear which Lazarus was intended). The Knights of Lazarus participated in battles, but they tended to all get killed...not very effective to send a bunch of leprous knights into battle, apparently.

In any case, the crusaders who were native to the east were more familiar with lepers and weren’t too disturbed by them, which is partly due to their memory of Baldwin IV as a courageous, heroic king.

Sources:

Bernard Hamilton, The Leper King and His Heirs (Cambridge University Press, 2000), and especially, as mentioned, Piers Mitchell’s appendix

Piers D. Mitchell, Medicine in the Crusades: Warfare, Wounds and the Medieval Surgeon (Cambridge University Press, 2004)

Piers D. Mitchell, “The myth of the spread of leprosy with the crusades”, in The Past and Present of Leprosy (Oxford, 2002), pp. 175-81.

Susan B. Edgington, "Medicine and surgery in the Livre des Assises de la Cour des Bourgeois de Jérusalem", in Al-Masaq 17 (2005), pp. 87-97.

Malcolm Barber, "The Order of Saint Lazarus and the Crusades", in The Catholic Historical Review 80, no. 3 (1994), pp. 439-456