A 38-year-old US man may have been infected with Zika virus through the tears or sweat of his dying father, researchers said, in what would be the first documented case of such transmission if confirmed.

The man in the western state of Utah became ill after helping to take care of his 73-year-old father, who was hospitalized in June with Zika after being infected during a trip to his native Mexico.

The known transmission methods -- being bitten by an infected mosquito, or sex with an infected person -- were ruled out for the son, according to a case study published Wednesday in The New England Journal of Medicine.

He had wiped his father's eyes and helped a nurse reposition him in bed without using gloves, the journal said. He never came into contact with his father's blood or other body fluids.

Tests had found an unusually high concentration of Zika virus in the father's blood, more than 100,000 times greater than levels found in other patients, according to the US Centers for Disease Control and Prevention.

The high level could explain "how the second patient may have contracted the virus by casual contact from the primary patient, the first such documented case," according to a news release from the University of Utah School of Medicine, where both patients were treated.

Researchers did not know why the Zika levels in the father's blood were so high, but speculated it could be because he had previously had dengue.

Some research has suggested that previous dengue infections could worsen a Zika infection, the Washington Post reported, citing Sankar Swaminathan, an infectious disease specialist at the University of Utah.

The father had also undergone radiation therapy one month earlier for prostate cancer, and was receiving anti-androgen hormone therapy but did not otherwise have a compromised immune system prior to falling ill with Zika, the case study said.

The father died after four days in the hospital, the first Zika-related death in the United States.

The son soon became sick with mild Zika symptoms but recovered after about a week.

Zika causes mild symptoms such as fever and rash for most people but pregnant women who are infected can give birth to babies with microcephaly, a deformation marked by abnormally small brains and heads.

"This rare case is helping us to understand the full spectrum of the disease, and the precautions we may need to take to avoid passing the virus from one person to another in specific situations," Swaminathan said in a University of Utah news release.

"This type of information could help us improve treatments for Zika as the virus continues to spread across the world and within our country," he said.