The virus plaguing both Korean and German infants began as a surprise. It surfaced during routine pregnancy medical examinations, and wasn’t detected until the day before birth. And it wasn’t a common flu bug. In each country, the culprit turned out to be a new virus that hit at the very tail end of the hormonally maddening final stage of pregnancy.

“That’s, to my knowledge, a first,” said Victor F. Hwang, a professor of pediatric infectious diseases at Washington University School of Medicine in St. Louis. “I would guess that this has never happened.”

In both countries, the usual shot for the day of birth was given to the mother and the suspected virus was found lurking in the blood or urine. The same drills began immediately. After the mother’s T-shirt was changed, nurses collected antibodies that are produced as a kind of “baby guinea pig” and tested at different times to check against similar viruses. The mothers were kept alert through the day of birth. The virus was discovered once the mother left the hospital and the baby went home.

Hwang and other experts said that, in fact, this is possible because the viral replication and reaction start so late that they normally go unnoticed until birth, if then.

The day after a woman gives birth to a child, clinicians examine the mother, the baby and a piece of clothing from her prior care. Cells from the baby’s throat are collected, and volunteers from the community are recruited to serve as human “trophies” by collecting antibodies from the breast milk. Microscopes and other testing equipment are installed in specialized rooms at the hospital, and medication is administered, according to processes similar to those for ordinary flu vaccine. In fact, the same teams responsible for administering regular flu shots were called upon.

“That would be a pretty good protocol,” said Jay Johnson, director of clinical research for The Vaccine Center at the American Association of Public Health Physicians. “Since the viral immune response could take up to one week before results were possible, even that rapid reaction is not uncommon,” he said.

Hwang noted that the virus that hit both the Korean and German infants appears to be a newer genetically different version of encephalitis virus, which might be more likely to show up in infants in both countries if the infected mother returned from a U.S. trip.

And, just as normal newborns would normally get the Hepatitis A vaccine, pregnant women and newborns in South Korea were given Hepatitis B vaccine prior to receiving Prenatal Immunization, considered one of the best ways to prevent polio and Hepatitis B infection in infants in the developing world.

The fact that the Korean mother and her baby were infected later than usual is probably as much a blessing as a curse for both nations. Had the virus been discovered earlier, perhaps the mother would have been treated with drugs, the baby would have been vaccinated with one of the all-important vaccinations and the mothers and babies would have ended up recovering well, possibly rather quickly.

“We’ve known about the problem for decades,” Hwang said. “But the problem has been persistent, which means people have not recognized the problem.”