An outbreak of Zika virus, a relatively new mosquito-transmitted infection, has caused substantial alarm in Brazil.

Since its appearance in spring of this year, thousands of people have become infected. Though the disease is annoying, it is not really dangerous, causing aches and fever and rash for a few days.

Zika, though, may be contributing to a much more substantial problem. Brazilian authorities are concerned that it is causing not just a self-limited disease but rather is profoundly affecting fetal development.

In 2015, the country has seen a greater than tenfold increase—from 147 cases to more than 2,400—of the devastating neurologic and developmental condition called microcephaly (literally “small head”). This neurologic calamity is characterized by incomplete brain development, and profoundly compromising intellectual and motor-sensory development.

The connection between the two epidemics is only circumstantial right now; indeed, the United States’ CDC regards it as “under investigation” because no cause-and-effect link is yet established.

The facts favoring Zika as the cause of microcephaly are this: The virus has been found in a few placentas of children born with the condition and, more compellingly, at autopsy of one baby with microcephaly who died. In addition, according to CNN, “most” of the mothers of microcephalic infants reported developing the symptoms of Zika early in their pregnancy.

Against Zika being the cause of the 2,400 infants with microcephaly is the brief history of the virus in humans. First described in 1947 in monkeys from Uganda’s Zika Forest, the disease was identified in humans in the 1960s.

To date, there have been a few large outbreaks. The first was in the Yap Islands in Micronesia, where a 2007 outbreak affected about three-fourths of the population, including dozens with well-documented active viral infection. Of note, no abnormalities were seen among babies born during and after the outbreak.

A second large outbreak occurred in 2013 in French Polynesia. Here, an estimated 28,000 of the 270,000 residents of the French territory were sick enough to seek medical attention; likely tens of thousands more had mild or symptomless infection. But once again, there were no descriptions of an uptick in microcephalic births. (And the diagnosis of microcephaly does not require medical training: The baby is born with a tragically cartoonish tiny head. The intense emotionality of such an event too increases the likelihood that local medical or public health authorities would be informed.)

Though Zika is a virus from an admittedly rough family—its genetic cousins include the viruses that result in yellow fever, West Nile, dengue, and chikunguna—none of these is known to cause fetal defects.

Why then, after Zika outbreaks in two previous areas with no reports of microcephaly, do we now see it in Brazil and a concurrent surge of infant cases of the condition? Viruses usually cause the same things here as they cause there despite the rapid genetic turnover of any pathogen. Therefore, the possibility is low that that the Brazil Zika has a previously microcephaly-causing genetic twist not seen before.

So perhaps the sheer scale of the Brazil outbreak, with overall cases likely to number in the hundreds of thousands, explains the spate of microcephalic newborns. Maybe Zika-microcephaly occurs only in 1 percent or fewer of pregnant women and the number of pregnancies occurring in the Yap Islands and Polynesia during the Zika outbreak was too low for the association to be seen.

Perhaps the two rare events are coinciding in time for still-uncertain reasons. Though this seems unlikely, it remains plausible given the lack of microcephaly cases in previous outbreaks. The implications for the country would be enormous—the smoking gun they are investigating may be a false first step and another cause, still not investigated, may be behind the condition. Plus, there are all of those headlines.

It may be too late for rational discourse though. Consider the ripple claiming that Brazil is calling on women to not get pregnant: This is a tough admonition and one even more difficult to trace to the source. To my read, a single infectious-disease practitioner, Dr. Angela Rocha, who, by her titles is not a public health or government employee, has made the suggestion.

It is unclear whether Dr. Rocha is speaking out of exhaustion since she is caring for so many with the condition or indeed is the person whom Brazilian authorities have identified as a spokesman (my fierce Google search failed to find out anything about her). No matter—this has become semi-official Brazilian policy, despite incomplete evidence on causality.

Welcome to the impossible world of public health decisions where leaders are damned if they do and damned if they don’t. The urge is always quite strong to do something, especially in the long shadow cast by the Ebola outbreak. Then, many felt the WHO and the CDC and other groups dawdled long after compelling evidence was in. And indeed the CDC and others have met Zika with a dull and sober “avoid mosquito bites” bromide given to everyone heading to areas with dengue or malaria or chikungunya—an intervention unequal to the tragedy unfolding in Brazil.

Yet to react to incomplete evidence creates a problem—to tell women not to get pregnant for a while (in a mostly Roman Catholic country and just months from the Summer Olympics, which will bring hyper-focus on the country) is a mighty big deal. Should Zika turn out to be unrelated to microcephaly, well, that is some advice that will be tough to explain.

So in this holiday season, as you are thanking those who make the world a better place, consider sending a little love not only to the panic-stricken families of Brazil but to the poor decision-makers who have to decide just how far to go to prevent another case of microcephaly. For them, tonight will be conspicuously lacking in dancing sugarplums.