Story highlights The Centers for Disease Control and Prevention is collecting semen from hundreds of U.S. men who had Zika

A second study in Puerto Rico is the largest collection ever of bodily fluids from Zika patients

(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.