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

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Edwards said the person had not traveled to a Zika-affected country and had not had sexual contact with anyone with the virus. But the individual had “helped provide care for the deceased patient.” Citing privacy, he and other officials declined to provide additional details.

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The primary mosquitoes known for carrying Zika are not present in Utah, Edwards noted. As a precaution, officials are trapping and testing mosquitoes around both persons' homes to definitively rule out that mode of transmission.

“At this time we don’t know if the contact between the new case and the deceased patient played any role in the transmission of the disease,” Edwards said. “There is uncertainty about how this new case contracted Zika. But we do not believe that there is risk of Zika transmission among the general population in Utah based on what we know so far."

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The Centers for Disease Control and Prevention on Monday said that the elderly man had traveled to a Zika-affected area and that subsequent lab tests showed he had uniquely high amounts of virus in his blood -- more than 100,000 times higher than that seen in samples of other infected people.

Public health investigators in Utah are interviewing the caregiver who was infected and additional family contacts to learn more about the types of interaction they had with the patient before he died. They also are collecting samples for testing, as well as working with facilities where that patient was to determine what contacts health-care workers there had.

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CDC Director Tom Frieden said in an interview that scientists are conducting genomic sequencing to determine whether there have been any mutations in the viral strain that infected the elderly man or the second person.

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Researchers are looking into the possibility that an infected mosquito may have traveled from the affected region back to Utah in the clothing or luggage of the man and then bitten the second person. Frieden said such infection, while rare, has occurred with malaria cases in Queens.

But the questions raised by the caregiver's case illustrate how little scientists know about the virus. Until the Zika epidemic exploded in Brazil last year, researchers had never seen a situation where a mosquito-borne illness causes birth defects, Frieden said. No other similar viruses are sexually transmitted as Zika is.

"This is a unique situation," he said. "This is the first time where there is a suspected spread of one person to another" beyond sex.

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Even if such transmission is confirmed, Frieden said it would be "unusual." He noted that the continental United States currently has more than 1,300 travel-related cases of Zika, and if this type of transmission is not rare, "it's likely that we would have seen this before."

The elderly man became very ill and was hospitalized at the University of Utah in Salt Lake City. The family contact cared for him at his home and at the hospital "and could have been exposed to that individual's sweat or tears or bodily fluids," Frieden said.

Researchers have detected Zika virus in blood, urine, semen, saliva and breast milk. It's also been found in genital tract swabs and fluid inside the eye, CDC officials said Monday. Scientists are investigating whether it is also present in sweat and tears.

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It's not immediately clear why the elderly man had such high levels of virus in his body, although he had an underlying disease that could have contributed, officials said. It's also possible that he had some "unique genetic susceptibility" unrelated to Zika, Frieden said.

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Officials said the second person showed symptoms of Zika but did not have the same high viral load that was present in the deceased patient.

With infectious diseases in general, people who are more severely ill and showing symptoms tend to be more infectious than individuals who aren't as sick.

If another type of person-to-person transmission is confirmed, Frieden said it would be hard to determine whether it has taken place in Brazil, the epicenter of the epidemic, because Zika has been so widely spread by infected mosquitoes there. Person-to-person transmission is much more likely to be identified in places like the United States, where there have not yet been documented cases of local mosquito transmission.

If normal routes of transmission are ruled out, "that leaves us with an unknown, possibly new mode of transmission," said William Schaffner, an infectious disease expert at Vanderbilt University Medical Center. He said it will be important to determine whether Zika can be transmitted through saliva, a known pathway for other diseases.

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Officials are interviewing and testing all the health-care workers who could have come into contact with the elderly man. That investigation is likely to take at least several weeks. At the moment, CDC does not plan to change its guidance for hospitals or health-care workers, Frieden said.

The CDC team includes two senior scientists with expertise in public health investigations, five epidemiologists, a public health advisor, two entomologists and others.

The man's death in late June was the first Zika-related adult death in the continental United States. Salt Lake County health department officials said the exact cause of his death has not been determined.

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Zika is spread primarily through the bite of an infected Aedes aegypti or Aedes albopictus mosquito. About 80 percent of infected individuals have no symptoms, while the rest tend to experience only mild problems that last for several days to a week.

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Symptoms include mild fever, skin rash, conjunctivitis ("pink eye"), muscle and joint pain, malaise or headache.

In rare cases, though, the virus has been linked to a nervous system disorder that can cause muscle weakness and sometimes paralysis. It poses its biggest danger during pregnancy, when infection can cause a range of severe fetal abnormalities.

In April, the CDC reported the first U.S. adult death from Zika in a patient in Puerto Rico. That man, who was in his 70s, died from internal bleeding after developing severe thrombocytopenia -- a rare immune reaction to his infection that causes low levels of blood-clotting platelets.

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Last week, the CDC and New York City health department reported for the first time that a woman had transmitted the virus to her male partner during sex. All previously reported cases of sexually transmitted Zika infection have been spread from men.

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As of July 13, no cases of locally transmitted, mosquito-borne Zika have been reported in the continental United States. As of July 13, a total of 1,305 cases of travel-associated Zika have been reported in the 50 states and District of Columbia. Federal, state and local officials are preparing for the possibility of local spread of Zika.

This post has been updated.