The main source for Baldwin IV's case of leprosy is a scholar named William of Tyre (1130-1186), who was a chronicler born to French parents in Syria and who was the king's tutor. His History of Deeds Beyond the Sea recounts how the young Baldwin first noticed numbness in his limbs, which was a dead giveaway for leprosy and was often at that time the first sign of the disease. All that William has to say about Baldwin's face is that he became disfigured and that people were moved to pity when they saw him. He does not mention any kind of a face covering.

If you'd like to see that text, consult the Internet Medieval Sourcebook: https://sourcebooks.fordham.edu/source/tyre-cde.asp#baldwin1

The case of young Baldwin does not appear to have provoked calls for his deposition as king. He was only thirteen when his father died, and passed away himself at twenty-three. The way I read it is that his courtiers/ministers did not have many useful alternatives to him as the heir to the throne (although he often had regents or other co-rulers appointed with him). He was succeeded by his sister Sibylla and her husband, Guy de Lusignan. The timing was terrible: Baldwin !V died just as Saladin was beginning his rise into the leadership of the anti-Crusader forces. There are many examples of royal heiresses who underwent multiple marriages to try to keep Jerusalem going as a kingdom.

On leprosy, its treatment and its stigma, I will append some bibliography below. Robert I. Moore, writing in the late 1980s, identified the period of Baldwin's reign, the 12th century, as an increasingly persecuting society--a society that actively excluded all groups that seemed to be "other"--including heretics, Jews, and lepers. Since Leprosy (Hansen's disease) was thought to be very contagious (it isn't), many medieval towns and cities built leper houses to isolate lepers away from their healthy people. Lepers were considered to be unclean, but medieval people gave money for their upkeep and seem to have regarded their plight as much a moral disease as a physical one. That way they could be excluded and stigmatized, but also pitied and helped, at the same time. Jesus, after all, healed a leper (Mark 1).

Here are some citations. I highly recommend Demaitre's exhaustive study:
Demaitre, Luke. Leprosy in Premodern Medicine: A Malady of the Whole Body. JHU Press, 2007.
Medcalf, Alexander, Monica Saavedra, Magali Romero Sá, and
Sanjoy Bhattacharya, eds. Leprosy: A Short History. York, UK/ Hyderabad:
Centre for Global Health Histories/ Orient Blackswain Private Ltd., 2016
Moore, Robert I. The Formation of a Persecuting Society. Oxford: Basil Blackwell, 1987.
Zimmerman, Susan. “Leprosy in the Medieval Imaginary.” Journal of Medieval and Early Modern Studies 38, no. 3 (September 21, 2008): 559–87. https://doi.org/10.1215/10829636-2008-007.
 

There is no mention of a mask in the medieval sources. Baldwin didn’t conceal his leprosy, and it was a cause of stigma against him - not so much in Jerusalem, but certainly among their Muslims neighbours as well as in western Europe.

Baldwin IV was the son of king Amalric of Jerusalem, who died in 1174, when Baldwin was about 13 years old. At the time, it was already suspected that he had leprosy. His tutor was the chancellor of the kingdom, the archbishop William of Tyre, who was also the court historian of Jerusalem and left us a detailed account of Baldwin’s life and reign. It was William who first discovered the possible symptoms when Baldwin was a child:

“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)

Usually when medieval people say “leprosy” we can’t really be sure what they mean; they were thinking of “leprosy” in the Bible (lepra in Latin and Greek, and tzaraat in Hebrew), which could have been leprosy in the modern sense, but also any other unrelated skin disease. They had no idea how leprosy was contracted, but they assumed it was sexually transmitted, or transmitted by any contact at all no matter how brief. Otherwise, theologically it was considered a physical sign of sin or God’s disfavour.

There were a few famous lepers in the Bible, notably Lazarus in the Gospels - well there were actually two Lazaruses, both of whom may or may not have had leprosy, but in the Middle Ages they were sort of conflated into one person with leprosy. In the Old Testament there was also a leper named Naaman who was cured by bathing in the Jordan River. Baldwin probably hoped people would see him as a Naaman rather than a Lazarus - when he was older, he even called himself “Naaman” in a letter to the king of France.

But aside from Naaman’s miraculous cure, there was no treatment at the time:

“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)

Consequently. there was an enormous social stigma against people with obvious signs of leprosy. According to Biblical law, lepers were supposed to be segregated from society, and the same restrictions were repeated in the medieval world. However, in the crusader world in the Near East, there were probably more lepers than there were back in Europe, so the stigma was not as strong. For example, the crusader military order of the Knights Hospitaller ran hospitals for lepers, and a sub-order, the Order of St. Lazarus, was founded for leprous knights. If Baldwin IV had been born in Europe he probably would have been segregated entirely and not allowed to rule.

Thanks to William’s descriptions we can even determine what kind of leprosy Baldwin had. In general, leprosy is caused by a bacteria, Mycobacterium leprae. It takes sustained contact (months or even years) with an infected person to spread, so it tends to spread among family members. We don’t know where Baldwin got it from, but presumably it was a family member, one of the other children he was playing with, or a wet nurse perhaps.

The bacteria itself is relatively harmless, but it causes various symptoms as the body tries to kill it, which is the real problem. Sometimes it develops into tuberculoid leprosy, where white blood cells attack and destroy the bacteria, but also damage tissue and cause inflammation, muscle weakness, and skin numbness. If the person can’t feel numb areas of their skin, they can develop ulcers, which lead to infections, and then bone and tissue damage and limb deformations.

The other possibility is lepromatous leprosy, where antibodies try to attack the bacteria cells; but since the bacteria ends up inside of a normal cell, antibodies can’t reach it. The bacteria cells grow to form disfiguring skin nodules (especially on the nose). But unlike tuberculoid leprosy, numbness and nerve damage (and therefore ulcers and infection) comes much later after the other more obvious symptoms occur. This is the most visible kind of leprosy and the kind we’re probably picturing when we think of a “typical” person with leprosy.

There are also other types of leprosy with symptoms in between these two types; one is polyneuritic leprosy, which has symptoms of numbness nerve damage but without the disfiguration and ulcers. The different kinds of leprosy can develop into other forms as well, due to age or stress. Lepromatous leprosy doesn’t turn into any other kind of leprosy, but polyneuritic leprosy, with the initial symptoms of numbness and nerve damage, can later turn into the lepromatous or tuberculoid forms.

Baldwin probably had the polyneuritic form of leprosy at first, which explains the numbness and the lack of disfiguration and ulcers. As he got older, he developed the symptoms of lepromatous leprosy. He lost the use his hands and feet, had skin ulcers and disfiguring growths, especially on his nose and face, and eventually he went blind as well. Lepromatous leprosy itself wouldn’t have killed him, but due to his ulcers and overall weakness, he could have easily developed something deadly:

“Possibilities include infectious diseases such as malaria, typhoid, a chest infection or perhaps septicaemia from an infected foot wound, common in untreated leprosy patients.” (Mitchell, in Hamilton, pg. 253)

But in 1174 none of this was clear yet; he had no obvious visible symptoms, so there was no objection to him becoming king, and everyone simply hoped he would be fine. The kingdom was governed by a council led by the count of Tripoli, Raymond III, but this was mostly because of his young age, not his disease.

His leprosy was no secret though. The Muslims believed Baldwin

“…was incapable of ruling. The Franks [crusaders] made him king in name with no substance to his position. The conduct of affairs was undertaken by Count Raymond with power of loosing and binding, whose command all followed.” (Ibn al-Athir, vol. 2, pg. 234)

Baldwin could obviously never marry and produce an heir - even if he had been physically capable, it would have been socially unacceptable, since leprosy was assumed to be sexually transmissible. Therefore the survival of the kingdom depended on his sister, Sibylla. As regent of the kingdom, Raymond III arranged for her to marry an Italian nobleman, William of Montferrat, in 1176. However William died the next year, leaving Sibylla pregnant with a child who was also named Baldwin, after his uncle the king.

By that time, Baldwin IV was legally able to rule on his own, as he was 15-16 years old, but Raymond continued to govern the kingdom. Although Raymond must have had a strong influence on military policy, it was Baldwin IV who led the army of Jerusalem in person in 1177 at the Battle of Montgisard, where he defeated Saladin, the sultan of Egypt.

A few years later in 1180, Baldwin suspected that Raymond was trying to overthrow him and seize the kingdom for himself, so he removed Raymond as regent and arranged another marriage for his sister, this time to a French crusader, Guy of Lusignan. Guy became regent, along with another Frankish aristocrat, Raynald of Chatillon, the lord of Oultrejordain (the land east of the Jordan River). But Guy and Raynald were unpopular with the other barons, some of whom believed they were needlessly provoking Saladin. They attacked and robbed Muslim pilgrims and caravans across the Jordan, and Raynald even tried to sail down the Red Sea and attack Mecca. In 1183 Baldwin dismissed them and resumed ruling on his own.

Later that year Saladin besieged the crusader fortress of Kerak, Raynald’s stronghold in the Oultrejordain. Once again Baldwin arrived in person, and Saladin retreated. But by then he couldn’t walk and had to be carried in a litter. It was around this time that Ibn Jubayr, a Muslim pilgrim from Spain, visited Jerusalem and noted that Baldwin no longer appeared in public:

“This pig, the lord of Acre whom they call king, lives secluded and is not seen, for God has afflicted him with leprosy. God was not slow to vengeance, for the affliction seized him in his youth, depriving him of the joys of his world.” (Ibn Jubayr, pg. 324)

So very informative! Thanks