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The Zika virus may spread in sweat and tears in some cases, doctors cautioned Thursday.

The case of a Utah man who infected his adult son before he died leaves no other alternatives, the team at the University of Utah School of Medicine said.

And — more bad news — the 73-year-old patient who died really was not very sick before he caught Zika, which suggests that the virus can occasionally kill people who are not frail and ill.

This is what a Zika rash looks like. Amit Garg, MD / Department of Dermatology, Hofstra Northwell School of Medicine

Dr. Sankar Swaminathan and colleagues describe the case in the New England Journal of Medicine.

The patient, who died in July, was the first in the 50 U.S. states to be killed by Zika. He’d been treated for prostate cancer but wasn’t especially ill from that, the team wrote in their report.

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“Eight days before admission, he had returned from a 3-week trip to the southwest coast of Mexico, where Zika virus transmission had been reported. He was well during his trip but reported being bitten by mosquitoes,” the team wrote.

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He developed muscle aches, diarrhea and other symptoms. The team thought he had dengue, a virus very closely related to Zika that’s spread by the same mosquitoes.

“Given the very high level of viremia in Patient 1, infectious levels of virus may have been present in sweat or tears."

He died from respiratory and kidney failure four days after being infected, they said. Later tests showed the older man in fact had Zika, and had an extraordinary amount of the virus in his blood — thousands of times more than usual. He'd had dengue in the past, but not recently.

Then his 38-year-old son got sick, and developed the rash that’s characteristic of Zika infection.

“Patient 2 reported having assisted a nurse in repositioning Patient 1 in bed without using gloves. Patient 2 also reported having wiped Patient 1’s eyes during the hospitalization but reported having had no other overt contact with blood or other body fluids, including splashes or mucous membrane exposure,” the team wrote.

Related: Zika Virus Mystery — How Did Man Infect His Son?

The younger man had not traveled, and the mosquitoes that spread Zika are not found in Utah. Investigators spent weeks trying to figure out how he got infected.

“Given the very high level of viremia in Patient 1, infectious levels of virus may have been present in sweat or tears, both of which Patient 2 contacted without gloves,” Swaminathan’s team concluded.

That's known to happen with Ebola, a different type of virus. When patients got extremely high levels of the virus in their blood, even their sweat became infectious to others.

"Those who are not severely immunocompromised or chronically ill may nevertheless be at risk for fatal infection."

Also like Ebola, Zika virus has been found in the eyes of patients.

“These two cases illustrate several important points. The spectrum of those at risk for fulminant (sudden and severe) Zika infection may be broader than previously recognized, and those who are not severely immunocompromised or chronically ill may nevertheless be at risk for fatal infection,” they said.

Related: How Can Zika Kill You?

“Whether contact with highly infectious body fluids from patients with severe Zika virus infection poses an increased risk of transmission is an important question that requires further research.”

Zika is mostly spread by mosquitoes, but the ongoing epidemic has shown it can also be spread through all forms of sex. The biggest danger is when a pregnant woman becomes infected, because the virus can get to the developing fetus and cause severe birth defects.

The virus can also cause a range of complications in some patients, including the paralyzing condition called Guillain-Barré syndrome, as well as inflammation of the brain and spinal cord.