Story highlights Zika can hide in a baby's blood for two months, causing later developmental delays

The virus lasts up to seven days in vaginal tissue in mice, impacting fetal growth

(CNN) More surprises from the world of Zika research this week: The virus can hide in vaginal tissue and the bloodstream of infected infants much longer than anyone expected, with dangerous consequences.

In most people, Zika lives in the blood for about seven days and then resolves. It can last a bit longer in urine and saliva, but even that goes away after a couple of weeks, leaving antibody testing of the blood as the only way to determine whether you've had the virus.

But a letter published Wednesday in the New England Journal of Medicine tells the story of a Brazilian baby who continued to have Zika in his blood for more than two months after his birth. He wasn't diagnosed with microcephaly when he was born, nor did he have any obvious developmental issues at his two-month checkup, even though his mother had symptoms of the virus during the third trimester of pregnancy.

But the baby's blood tested positive for Zika 54 days after birth and then again at day 67, and by the time he turned 6 months of age, he had severe muscle contractions and other signs of neurological decay

So when was the baby's brain damaged, before or after birth? No one knows. The case also raises questions about how long the virus can hide within the body and where.