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After the deaths of two children who contracted the herpes virus through an ultra-Orthodox practice of circumcision, the New York City Board of Health voted today to require parents to sign a written consent that warns them of the dangers.

In the most controversial part of this version of the Jewish ritual, known as metzitzah b'peh, the practitioner, or mohel, places his mouth around the baby's penis to suck the blood to "cleanse" the wound. The city wants parents to know the risks before circumcision.

The more than 5,000-year-old religious practice of circumcision is performed during a Jewish religious ceremony known as the bris, which is observed by Jews of all denominations around the world.

The modern Jewish community uses a sterile aspiration device or pipette to clean the wound in a circumcision. About two-thirds of boys born in New York City's Hasidic communities are circumcised in the oral suction manner, according to Rabbi David Zwiebel, executive vice president of the Orthodox Jewish organization Agudath Israel of America.

Some estimate that 70 percent of the general population is infected with the Type 1 herpes I (HSV-1), which can be transmitted from the mouth to the child. It is different from Type 2 or genital herpes (HSV-2), which is a sexually transmitted disease and can cause deadly infections when a newborn passes through an infected birth canal.

Neonatal herpes infections of all kinds are nearly always fatal in infants.

In 2003 and 2004, three babies, including a set of twins, were infected with Type 1 herpes; the cases were linked to circumcision, and one boy died. Another died in 2010. In the last decade, 11 babies in the city have contracted the virus, and two have had brain damage, according to health officials. All were circumcised by the metzitzah b'peh method.

The health department has said there is no safe way to perform oral suction on an open wound in an infant.

But some rabbis have said that they will oppose the law on religious grounds, insisting it has been performed "tens of thousands of times a year" worldwide. They say safeguarding the life of a child is one of the religion's highest principles.

"This is the government forcing a rabbi practicing a religious ritual to tell his congregants it could hurt their child," Rabbi David Niederman, executive director of the Hasidic United Jewish Organization of Williamsburg, told ABCNews.com. "If, God forbid, there was a danger, we would be the first to stop the practice."

But Dr. William Schaffner, chair of preventive medicine at Vanderbilt University, told ABCNews.com during the investigation of one of the deaths last spring, "It's certainly not something any of us recommend in the modern infection-control era."

"This is a ritual [of oral suction] … that's come down through the ages, and now it has met modern science," he said. "It was never a good idea, and there is a better way to do this."

The Department of Health argues parents should be informed of the risks before making a decision. Since 2004, it has received "multiple complaints from parents who were not aware that direct oral suction was going to be performed as part of their sons' circumcisions," according to a public notice.

Dr. Jay K Varma, deputy commission for the Dept of Health, admits the law will be difficult to enforce, but will enhance public information about the risks of oral suction.

"We certainly hope the force of law and will get people to want to comply. But as with other things in life, we can't 100 percent verify that. We realize it is not practical to monitor every circumcision in the city and see that consent is delivered. It's simply not practical for us and we don't have the resources."

The law will require mohels who perform oral suction in circumcisions to explain the risks to parents prior to the procedure, including the possible transmission of herpes simplex virus, and have parents sign a waiver.

With reporting by Katie Moisse.