(CNN)The Centers for Disease Control and Prevention is collecting semen from hundreds of men in the United States in order to figure out how long the dreaded Zika virus lasts in the bodily fluid.
The virus can be transmitted sexually, and it's been known to linger in semen long after a man's fever, rash and itchy eyes have gone away.
If a man has the virus in his semen and has sex with a woman who is pregnant or becomes pregnant, the baby could be born with devastating neurological birth defects.
After about two months of recruiting, some 40 men who've had Zika have volunteered to donate their semen. The CDC hopes to bring in about 210 more.
The men are asked to make about a dozen donations in their homes every other week for six months after their illness, and each time are given a $50 multi-use gift card.
A courier picks up the donations, which are then delivered to the CDC's labs in Fort Collins, Colorado.
"I'm happy to say patients really have been quite receptive about volunteering their specimens," said Dr. Paul Mead, the senior epidemiologist at the CDC who is running the study. "They seem to understand the importance of the study."
That advice is based on studying the semen of just three men who had Zika, which is why the CDC is doing the larger study.
Separately, the CDC is doing a study in Puerto Rico that will examine the semen of men who contracted Zika but never actually had symptoms of the disease
-- an important group since many who become infected have only mild symptoms or none at all, according to the CDC.
In addition to looking at the virus in semen, the Puerto Rico study will also be examining blood, saliva and vaginal secretions.
The CDC plans to recruit 300 men and women in Puerto Rico who've had Zika and 1,000 people who live in the same households.
"The response has been amazing. People have welcomed strangers to come and get their bodily fluids," said Dr. Gabriela Paz-Bailey, the investigation's lead researcher.
She said the Puerto Rico study is the largest collection ever of bodily fluids from Zika patients.
"Nearly everyone we've approached has agreed to participate," Paz-Bailey said. "They understand that many things are not yet known."