About 18 million people have contracted onchocerciasis, almost all of whom live in Africa, with a few scattered cases in Venezuela and Brazil. Onchocerciasis is not fatal, but it results in a miserable life. The disease has blinded around 270,000 people alive today. In Liberia, infected workers on a rubber plantation have been known to place their machetes in a fire pit and then use the red-hot blades as a tool to relieve the relentless itching. Of course the itching also makes sleep elusive, as Katabarwa explains: “Children with the worms can’t concentrate because they are scratching themselves all day and all night.” Suicide is common among its victims.

While there is no vaccine for onchocerciasis, it can be controlled with a drug called ivermectin, which has been donated worldwide by the pharmaceutical firm Merck since 1985. Treatments with ivermectin every six months kill newborn worms (called microfilariae), which releases the itch-triggering bacteria in their guts all at once. While this results in a two-day bout of itching that is even more excruciating than that in a normal case, sweet relief follows this brief episode. Semanza was fortunate to receive ivermectin in a locally administered program initiated by Katabarwa. Two years after he began treatment, the itching was gone, his skin was partially healed, and he was reintegrated with his community, married, and hoping to start a family.

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Itching can be a brief sensation or it can last for days. In the case of untreated onchocerciasis, it can endure for a lifetime. It can be triggered by mechanical stimuli, like a wool sweater or the subtle movement of an insect’s legs over the skin, or by chemical stimuli, like the poison-ivy inflammatory agent called urushiol. Itching can also result from damage to sensory nerves or the brain. In some cases it can be triggered by brain tumors, viral infection, or a mental illness like obsessive-compulsive disorder. It’s also a well-known side effect of certain therapeutic and recreational drugs.

Itching is highly subject to modulation by cognitive and emotional factors. One night, camping in the Amazon jungle, I was just drifting off to sleep when I felt an itchy sensation on my arm. I got my flashlight and glasses, saw what was causing it, and brushed off a huge millipede. At that point, sleep became impossible. I had become hypervigilant, and every little breeze and twitch evoked a sensation of itch for the rest of the night, not just on the affected arm but all over my body. I was battling millipedes of the mind until dawn.

The compelling, tormenting nature of itch is well known. In Dante’s Inferno, falsifiers (including alchemists, impostors, and counterfeiters) were cast into the Eighth Circle of Hell, where they suffered eternal itch. Only those who committed treachery—fraudulent acts between individuals who shared special bonds of love and trust (like Judas Iscariot, the betrayer of Jesus Christ)—met a presumably worse fate in the Ninth Circle of Hell: being frozen in ice.