Zika virus infection appears to have passed from one member of a Utah family to another, but not by any of the known paths of transmission: sex, pregnancy, or mosquito bite. The case, announced Monday by the Utah Department of Health and the Centers for Disease Control and Prevention, reveals that the risks posed by the novel, fast-moving virus are still not understood.

The agencies said in back-to-back press conferences that a “family contact” of an elderly man who died in June—and who was discovered after his death to have been the first Zika-related death in the continental U.S.—has also come down with the disease. That person, who helped care for the ailing man, experienced mild Zika symptoms of rash and fever and has recovered, the officials said. (The agencies declined to identify the family member; the Wall Street Journal reports it was his son.)

The first patient, the elderly man, traveled to a place where Zika is circulating and was probably infected there, the officials said. But trapping and testing by Utah mosquito-control teams, which operate in warm-weather months to search for West Nile virus, have not uncovered either of the mosquito species believed to transmit Zika—meaning it is unlikely the dead man passed the virus to a mosquito which then passed it back to a human.

Gary Edwards, a health officer in the Salt Lake County Health Department, said in the Utah press conference: “This case is unusual. The individual does not have any of the known risk factors we’ve seen thus far with Zika.”

'Uniquely High Amounts' of Virus

The lack of other explanations poses the possibility that the family member became infected in a manner that scientists have not yet documented, possibly by exposure to bodily fluids. The virus has been found in blood, semen, urine, saliva, cervical mucus, breast milk, and the fluid inside the eyeball. The CDC has been warning blood banks to act as though Zika can be transmitted by transfusion, though there have been no transfusion cases identified so far.

“We are trying to determine if the contact between the very sick, elderly patient and the person played a role in how the person got sick,” Satish Pillai, a physician involved in the CDC’s Zika response, said during the press briefing. “We don’t have all the answers right now.”

The elderly man had an underlying health condition that officials have not described; when he died, officials were careful to say his demise was associated with Zika but not necessarily caused by it. The investigation into his death revealed that he had what the CDC called “uniquely high amounts” of virus in his blood, 100,000 times higher than has been recorded in other cases that have been analyzed.

That could indicate that the man’s blood, and possibly other bodily fluids, was uniquely infectious, because it carried so much more virus than usual. But federal officials said Monday that it’s not yet possible to know whether it was his underlying illness or a genetic factor or even the strain of Zika infecting him that allowed the virus to replicate to such high numbers in his system.

“A high viral load is something we take very seriously, and it is not something about which we have a very long experience,” Michael Bell, a physician and deputy director of the CDC division that handles infection prevention in hospitals, told reporters. “Someone who is extremely ill and debilitated from another disease process could have a diminished immune system that doesn’t fight the virus as well. On the other hand, someone with a high viral load could be more sick as a result of the actual viral infection.”

A Preexisting Link?

Before his death, the elderly man experienced thrombocytopenia, a disorder that can be triggered by infections and causes bleeding in internal organs, in the mouth and nose, and from nicks and cuts. Last week, physicians in Puerto Rico reported that two Zika patients there also developed thrombocytopenia, and one “died following multiple hemorrhages.”

That person, a 72-year-old man with high blood pressure, was the first person in the greater U.S. to die from a syndrome related to Zika. Other cases and deaths from blood disorders linked to Zika have been reported in Colombia, Suriname, and Polynesia. The physicians who treated the Puerto Rico patients warned this is something to be wary of—especially since both men were misdiagnosed with dengue and not identified as having Zika until the bleeding disorder began.

Asked whether an association between Zika infection and body fluids would be any cause for alarm, CDC officials pointed out that there have been 1,306 known cases of Zika in the U.S. so far, and only a few have involved person-to-person transmission. (That includes the revelation last Friday that Zika can be passed not only from male sexual partners to women, but from women to men as well.) Bell, from the CDC, said that hospital workers should already be protected because healthcare institutions all practice the “standard precautions” against body-fluid contact that have existed since the start of the AIDS epidemic.

“Currently we are not altering PPE [personal protective equipment] instructions,” the CDC’s Bell said. “What this highlights is the fact that when you have an infection like Zika, wherein a good percentage of patients don’t actually have symptoms, it means it is as important as ever to stick with good adherence to standard precautions … It is a great example of why we should never take chances.”