Another Zika mystery is unfolding in Utah.

Local and national officials said Monday they were surprised to learn that an elderly Utah resident, who died in late June after a bout with Zika, may have infected a family member who cared for him during his illness. The circumstances of the second infection raise thorny questions about possible new routes of transmission for Zika, which has been known to spread via mosquito bites and sexual contact. Officials have also not been able to confirm whether or how the virus may have caused the patient’s death.

The deceased had acquired Zika while traveling to an affected area, and lab tests confirmed an unusually high concentration of virus in his blood: more than 100,000 times higher than what has typically been found in other infected people, according to the U.S. Centers for Disease Control and Prevention. “This is a unique situation with these elevated viral loads that we haven’t previously seen,” says Satish Pillai, incident manager for the CDC’s Zika Response. At this point the CDC says there is no new recommendation about how family members should care for a Zika patient nor answers about what may have caused this patient’s high viral load.

Federal, Utah and Salt Lake City County public health officials are investigating how the caregiver, who is also in Utah, may have contracted the illness. It is unclear if the person could have become infected via contact with the infected patient’s bodily fluids or if a mosquito may have bit the infected individual and transmitted it to the family member. In past studies Zika virus—or parts of it—have been detected in fluids including blood, semen, saliva, breast milk, internal eye fluid and urine. To date there is no evidence that the mosquito species that commonly spread Zika are in the Utah area. There is also no evidence that this second patient, who has now recovered, had sexual contact with anyone infected with Zika. 

The CDC sent an emergency response team to the state at the request of the Utah Department of Health, and family members and health care workers are being interviewed. The CDC says responders are trapping mosquitoes around the two patients’ homes to help answer questions about whether local mosquitoes may transmit the virus. They are testing the mosquitoes for both Zika and West Nile virus. It is “extremely, extremely unlikely” that the virus was spread via some form of airborne transmission, says Michael Bell, deputy director of CDC’s Division of Healthcare Quality Promotion.

Although more than 1,300 people in the continental U.S. and Hawaii have been infected with Zika via travel to affected areas or sexual contact with someone who had contracted the virus, there has been no known instance of someone getting it through a mosquito bite while in the continental U.S.