A Utah man who mysteriously contracted Zika from his father may have been infected by touching his dad’s tears or sweat with his bare hands, according to new research. The study suggests the unusual transmission may be linked to the fact that the man’s dying father carried 100,000 times the normal level of the virus.

The research, by University of Utah scientists, was published Wednesday in the New England Journal of Medicine. It doesn’t answer why the father’s levels were so high or why the son contracted the virus in a way not documented anywhere else, but it does offer new details about the events that led to the puzzling case.

The father, 73, had been diagnosed with prostate cancer eight months before his June death, the report says. He was receiving radiation therapy and anti-androgen therapy, which may have made it easier for the virus to replicate, said Dr. Sankar Swaminathan, chief of the infectious diseases division at University of Utah Health Care. But Swaminathan said the cancer and treatment doesn’t totally explain why the man had such high levels of the virus.

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The father became ill after returning from a three-week trip to the southwest coast of Mexico​, his native country. He’d moved to the United States in 2003, the paper said. The journal report doesn’t provide the man’s name or the location he visited in Mexico.

When he was there, he ate ceviche and soft boiled turtle eggs and went fishing in the ocean, but those food choices and activities played no role in his infection, Swaminathan said. He and other family members were bitten by mosquitoes while there and infected with the Zika virus

His son, a healthy 38-year-old, became sick five days after visiting his father in the hospital and was diagnosed with Zika, too. He recovered and later told doctors that he had helped nurses care for his father, including wiping his eyes without gloves.

None of the nurses and doctors who treated his father became sick, nor did other family members. Health officials and researchers tested mosquitoes in the Salt Lake City area and didn’t find any that were Zika-infected.

The virus causes only mild illness in most people​. But scientists have linked infection during pregnancy with severe brain-related birth defects​, including microcephaly​.

The authors of the new report concur with a previous assessment by the U.S. Centers for Disease Control and Prevention that suggested the son contracted the virus through a new method other than the two previously identified modes of transmission: being bitten by a Zika-carrying mosquito and sexual activity with an infected person.

Swaminathan said people visiting Zika-infested areas and countries don’t need to wear gloves. They believe the rare transmission happened primarily because of the father’s extremely high levels of the virus.

“There’s no risk of shaking hands with a person who has a typical Zika infection,” said Swaminathan, who treated the two men.