Research suggests a Utah man who contracted Zika from his infected father might have got it by touching his dad's tears or sweat with his bare hands.
The research by University of Utah doctors and published on Wednesday in the New England Journal of Medicine says the unusual transmission was likely caused by the dying father having 100,000 times the normal level of the virus.
The report does not give definite answers to why the father's levels were so high or why the son contracted the virus in a way not documented anywhere else, but it does offer new details about the case that has puzzled researchers.
They say the 73-year-old man who died had been treated for prostate cancer and had just returned from a trip to the southwest coast of Mexico.
US health officials have concluded that Zika infections in pregnant women can cause microcephaly, a birth defect marked by small head size that can lead to severe developmental problems in babies.
There is no vaccine or treatment for Zika.
