A case of the Zika virus in Utah is now the only one in the continental U.S. that's still puzzling researchers on exactly how it spread, health officials said Tuesday.

The man caught the illness after caring for his infected father, who had an extremely high level of the virus in his blood when he died in June, according to a report released by the U.S. Centers for Disease Control and Prevention. One possibility is that he transmitted the virus to his son through a bodily fluid in a way that hasn't been recognized with Zika yet, officials said. The son kissed and hugged his dying father and helped care for him in a hospital, according to the report.

Investigators couldn't test the unidentified Salt Lake County man because he had already been cremated by the time he was diagnosed, said Angela Dunn with the Utah Department of Health. Signs of Zika have been found in blood, urine, semen and saliva, and the case could direct new research into whether it can also be carried in things like tears or stools, she said.

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No other cases have yet been found among the family or health workers. It's unclear whether the case points to a potentially common mode of transmission, said Alex Kallen with the CDC. "I don't think we know the answer to that right now," he said.

The father caught the mosquito-borne virus abroad, but the son didn't travel to an affected area or have sex with anyone who did, the report said.

The son has since recovered. The virus causes only a mild illness in most people. But during recent outbreaks in Latin America, scientists discovered that infection during pregnancy has led to severe brain-related birth defects.

Health officials also haven't found any of the tropical mosquitoes that mainly spread the virus in the area or evidence that local mosquitoes are carrying the virus, according to the report released by the CDC.

The son didn't report touching any bodily fluids, and other family members had similar interactions with the father without getting sick, Dunn said. "We weren't able to identify anything he did that was different," she said.

It's not uncommon to be unable to identify a key thing a patient did or didn't do before catching an illness like Zika, Kallen said. Small, even unremembered, factors can often make a big difference, he said.

The investigation is ongoing, and more information is expected later this year.

The father died of septic shock with multiple organ failure, the report states. His was the first Zika-related death in the continental U.S.

The CDC report cautions other family caregivers that the virus can be spread by blood and bodily fluids of Zika patients.

More than 2,900 Zika illnesses have been reported in continental U.S. and Hawaii, including 13 in Utah, according to health officials.