"Some of these communities had never seen a doctor before," he said. "We started along the coast and worked our way into the interior."
By 1998, the medical mission reached Bartica, the last semblance of urbanity for gold and diamond hunters heading deep into the bush. In the surrounding communities, reachable only by boat, Dr. Silverman saw the infection on the native children for the first time.
Children covered in sores
Ms. Harper, now 27, knew from the outset it would be complicated.
Syphilis belongs to a family of Treponema bacteria that includes three other subtypes that cause skin infections, but only T. pallidum, the syphilis bug, is sexually transmitted.
Untreated, all Treponema infections can leave telltale lesions on the bones, and a few reports of scarring found on the skeletons of pre-Columbian remains has suggested syphilis might always have existed in Europe. But Ms. Harper said there is no way to know if such bone lesions were the result of syphilis or other Treponema skin infections, such as yaws.
At the same time, Treponema bacteria don't grow easily in a lab and not many are available for study. Only five labs were known to keep strains of yaws. Meanwhile, yaws was believed to have vanished from most of the world, and it had not been seen anywhere in the Americas for decades - until Dr. Silverman discovered it in Guyana.
The children were hard to miss, creeping through the villages like lobsters with their sore-covered limbs and infected bones, bent and bowed. The maiming infection had spread among children playing sleeveless and bare-legged.
But still it nagged him - how could this be yaws? During the 1950s and 1960s, the World Health Organization had waged a treatment campaign against yaws. In 1985, he recalled, a report had declared the disease eradicated in the Western hemisphere.
"There had been no reported cases since the '70s," he said.
Nor did the disease look like textbook cases. Instead of the hallmark raspberry-shaped sores of yaws, the Guyanese children were riddled with round, red ulcers.
"It looked like syphilis!" he thought. For a moment, Dr. Silverman was struck sick. Had he stumbled into a widespread network of child sex abuse? But that made no sense. "These children had sores on their shins and elbows, not where you would expect to see them if it was syphilis."
So what was it? And how could he find out without power or proper lab equipment, in a remote patch reachable only by boat and 16 kilometres of portaging?
The Ve'ahavta team dragged generators up Guyana's muddy slopes, jumped them with battery cables and strapped flashlights to their heads. The blood analysis confirmed Dr. Silverman's first assumption - yaws.
Further study revealed the disease was endemic in the region, infecting 5.1 per cent of children. But by 2001, after the Canadians gave the children penicillin tablets, the incidence dropped to 1.6 per cent and the numbers kept shrinking.
In 2003, Dr. Silverman and his colleagues reported the success of their South American yaws control program in an issue of the journal of Clinical Infectious Diseases that Ms. Harper happened to read.
"It was such a lucky coincidence that I saw it," she said.
In the winter of 2005, Ms. Harper reached Dr. Silverman on his cellphone; he was at Toronto's Pearson airport on his way to Guyana.
"She sounded really young and she told me she loved our work," Dr. Silverman recalled. "I said I was in airport security and it wasn't the best time."
"You know you're the only person to have seen yaws in the Western Hemisphere for about 35 years?" she said.
"I hope to be the last," he said, "We're heading down there right now. ... We're trying to make it extinct."
Ms. Harper explained her study and asked Dr. Silverman to collect bacterial samples from the Guyanese children. He said he didn't know if any children would still be infected - but he promised to try.