The most common form is cutaneous (i.e., skin) leishmaniasis, which is found in many parts of the Old World, especially Africa, India, and the Middle East. It is also widespread in Mexico and Central and South America, and it recently popped up in Texas and Oklahoma. Some US troops returning from Iraq and Afghanistan contracted cutaneous leish during their deployments and nicknamed it the Baghdad boil. This kind of leish starts as a sore at the location of the bite, which grows into a weeping lesion. If left alone, it usually goes away, leaving only an ugly scar. It can usually be treated by burning, freezing, or surgically removing the ulcer.

In visceral leishmaniasis, the second type—also from the Old World—the parasite invades the body’s internal organs, particularly the liver, spleen, and bone marrow. It is sometimes known as black fever, because it often turns its victim’s skin black. This variety is deadly; without treatment it is always fatal. But treatment of visceral leish is quick and reliable, requiring a single infusion of an antibiotic drug that yields a cure rate of about 95 percent. Most of the deaths in the world from leish are caused by the visceral form, among poor children who don’t have access to treatment.

The final form of leish is the mucocutaneous or mucosal variety, the major New World form of the disease. It starts as a skin sore like the cutaneous kind. Months or years later, the sores can reappear in the mucous membranes of the nose and mouth. (The sores I had in my mouth, however, were probably unrelated.) When leish moves to your face, the disease gets serious. The ulcers grow, eating away the nose and lips from the inside and eventually causing them to slough off, leaving the face horrifically disfigured. The parasite continues to devour the bones of the face, the upper jaw, and teeth. This form of leish, while not always lethal, is the most difficult to treat, and the treatment itself involves a drug that has toxic—and sometimes fatal—side effects.

The pre-Columbian inhabitants of South America were plagued by mucosal leish, which they called uta. The grotesque disfigurement of the face terrified the Moche, Inca, and other ancient cultures. They may have considered it a punishment or a curse from the gods. Archaeologists have uncovered burials in Peru and elsewhere of people whose disease was so advanced that they had a caved-in hole where the face used to be—the disease had eaten away everything, including the facial bones. Ancient Peruvian pots so faithfully record the disfigurations that researchers can identify in them the actual clinical stages of the disease, from the early soft-tissue destruction of the nose, to the general destruction of the nose and lips, and finally to the disintegration of the hard palate, nasal septum, upper jaw, and teeth. The Peruvian custom of punishing people by mutilating the nose and lips may have been intended to imitate the facial deformities caused by the disease, perhaps to mimic what they believed was divine retribution.

Acute fear of the disease may have even driven human settlement patterns in South America. The archaeologist James Kus, a retired professor at California State University, Fresno, believes that the Inca site of Machu Picchu may have been chosen, in part, because of the prevalence of mucosal leish. “The Incas were paranoid about leishmaniasis,” he told me. The sand fly that transmits leish can’t live at higher altitudes, but it is widespread in the lowland areas where the Inca grew coca, a sacred crop. Machu Picchu lies at just the right altitude: too high for leish, but not too high for coca; at Machu Picchu the king and his court could rule from a place of safety and preside over the rituals associated with coca cultivation, without the risk of getting this most dreaded disease.

When the Spanish conquistadors arrived in South America in the sixteenth century, they were horrified at the facial deformities they saw among native people in the lowlands of the Andes, especially among the coca growers. The Spanish thought they were looking at a form of leprosy and called the disease lepra blanca, “white leprosy.” Over the years, mucosal leish has acquired many nicknames in Latin America: tapir nose, hoarse voice, spongy wound, big canker.

Mucosal leish didn’t exist in the Old World. But the even deadlier visceral form, the kind that invades the internal organs, had long plagued the Indian subcontinent. It first gained the attention of Western medicine as the British extended their empire into India. Eighteenth-century writers described it as “kala-azar” or “black fever.” Visceral leish easily spreads from person to person via the bite of sand flies, using human beings as its primary reservoir host. It was so deadly and spread so fast that in certain regions of India in the nineteenth century, leishmaniasis would sweep through an area, killing everyone and leaving a landscape of empty villages, entirely bereft of human life.

The British also noted the cutaneous form of the disease in India and the Near East and gave it various names: Aleppo evil, Jericho button, Delhi boil, Oriental sore. But doctors did not recognize a connection between the two strains until 1901. William Boog Leishman, a doctor from Glasgow who was a general in the British Army, was posted in the town of Dum Dum, near Calcutta, where one of his soldiers fell ill with a fever and a swollen spleen. After the man died, Leishman looked at thin sections of the man’s spleen under the microscope and, using a new staining method, discovered tiny round bodies in the cells—the leishmania parasite. Leishman called it Dum Dum fever. A few weeks after Leisham published his discovery, another British doctor, named Charles Donovan, also stationed in India, independently reported the results of his own research. He, too, had spied the offending parasite, and between the two of them the disease “leishmaniasis” was identified. Leishman got the dubious honor of having the disease named after him, while Donovan was gifted with the species’ name: Leishmania donovani. Doctors figured out in 1911 that it was transmitted by the sand fly, and later they realized that a bewildering number of mammals could be reservoir hosts for the disease, including dogs, cats, rats, mice, gerbils, hamsters, jackals, opossums, foxes, monk seals, and, of course, humans. This astonishingly broad range of host animals makes it one of the most successful diseases on the planet.