A Utah man who became the first person in the continental US to die after being infected with the Zika virus passed it to a caregiver, creating a medical mystery about how it spread between them, health officials said on Monday.

The two people did not have sexual contact and the type of mosquito that mainly spreads the virus is not found in the high-altitude area where they live, the Salt Lake County health department said.

The caregiver is a “family contact” and has fully recovered, but officials did not give further details, including how the virus was transmitted.

“Our knowledge of this virus continues to evolve and our investigation is expected to help us better understand how this individual became infected,” said Dr Angela Dunn, deputy state epidemiologist at the Utah department of health.

“Based on what we know so far about this case, there is no evidence that there is any risk of Zika virus transmission among the general public in Utah.”

The man who died in late June caught the virus while traveling abroad in an area where mosquitoes are known to spread Zika; he had an unusually high level of the virus in his blood, officials said. The exact cause of the death announced on 8 July was not clear because the man was elderly and also had an underlying health condition.

He had Zika symptoms – including rash, fever and pink eye – but it was unclear if or how the virus contributed to his death, the Centers for Disease Control and Prevention (CDC) said. The transmission to the caregiver was discovered after a doctor noticed Zika-like symptoms.

Utah authorities have refused to release additional information about the man who died or where he traveled, citing health privacy laws.

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. It has spread mainly through the bite of a tropical mosquito.

No cases of mosquito-spread Zika have been reported in the continental US, according to the CDC. Health experts think mosquito transmission probably will occur in the US, but the expectation is that it will be in low-elevation, sweltering places where the insect has been a steady problem – like southern Florida or southern Texas.

More than 1,300 Zika illnesses have been reported in the 50 states and the District of Columbia, including eight in Utah, according to health officials. Almost all were people who had traveled to Zika outbreak countries and caught the virus there. But 14 were people who had not traveled to Zika zones but had sex with someone who had.

The CDC has also been tracking pregnant women infected with Zika, and says it has five reports of pregnancy losses because of miscarriage, stillbirth or abortion.