This is disturbing and sad. A tropical disease that is spread by mosquitoes and that seems to be moving north—and may be approaching the US border—is being linked in health-agency reports to a disabling birth defect.

The disease is Zika virus, and the birth defect is microcephaly: smaller than normal heads in infants, due to the brain having stopped growing at some point in pregnancy. According to the World Health Organization, Brazil has reported 739 cases of microcephaly among newborns in nine states in the “shoulder” of Brazil, the northeastern corner that protrudes into the Atlantic. Brazil’s Ministry of Health says there are more: 1,248 suspected cases, in 14 states. Seven children have died, and five others’ deaths are being investigated, according to the Ministry; one adult and one teenager have also died.

Only one child, who died, has been definitively diagnosed with the virus by tissue analysis, according to the Ministry’s statement. The agency has declared a national public health emergency and, in its most recent statement, says it has asked the US Centers for Disease Control and Prevention to send a team to help.

Microcephaly can be caused by many things; according to the US National Institute of Neurological Disorders and Stroke, they include exposure to toxins during pregnancy, excessive drug or alcohol use, and a range of infections such as rubella and chickenpox. And it’s not unknown in Brazil, but according to the Ministry’s web site, the country has had only between 139 and 175 cases per year in each of the past five years—so this spike, whatever is causing it, is almost 10 times as many.

In a sign of how much concern these cases are causing—as well as how much travel to South American beaches occurs in winter months—the European Center for Disease Prevention and Control released a “rapid risk assessment” on Friday. It cautions, “There is currently only ecological evidence of an association between (the virus and the birth defects),” but adds, “a possible causative association cannot be ruled out.” Pregnant women, the EU agency said, should be careful to wear insect repellent and long pants and sleeves, and should try to stay inside behind screened windows.

View Images A comparison of microcephalic birth-defect cases by year in Brazil over 6 years. Graphic by the ECDC, original here.

Meanwhile, Zika virus has only been present in Brazil since April, according to the Pan American Health Organization, and it is new in the Americas, having arrived in Chile just last year. It has a long history in Indonesia and the Pacific—including islands such as Vanuatu, Samoa and the Solomon Islands—and in West Africa. Evidence of past infections, though not current cases, has been found in India and in East Africa as well. It is carried by the mosquito species Aedes aegypti, which is the same mosquito that transmits dengue and chikungunya.

In most of the areas where it has been for a while, Zika causes only mild illness of fever, red eyes, and joint pain—but the joint pain is not so serious as what is caused by dengue, dubbed “breakbone fever.” Zika is rarely fatal. That’s one reason why the Brazil cases have caused such alarm. But in French Polynesia, according to the ECDC, there have recently been “at least 17 cases” of malformed brains in children conceived or born last year and this year. None of the women remembered experiencing any symptoms of Zika; but tests on four of the mothers showed they all had been infected with the virus without knowing it. And separately in Brazil, there has been a spike in cases of Guillain-Barré paralysis in the states where the birth defects, and Zika, are also happening.

Though Zika is a new arrival in the Americas, it is moving quickly, and that’s where North Americans should be concerned. According to the WHO, cases have been found in Suriname, Colombia, and just in the past few weeks, El Salvador and Guatemala—in other words, moving north through Central America.

And, possibly, further than that: Last Thursday, a Mexican regional newspaper, the Cambio de Michoacán, reported that Mexico’s Minister of Health announced two locally infected cases of Zika in the country. One was in Chiapas, on the border with Guatemala; the other was in the state of Nuevo Leon, which lies close to, and at one point touches, the United States’ border.

(Mexico also had its very first case, in a man who was infected in Colombia, just one week earlier; Crawford Kilian spotted and translated the report. Crof, who lives in Canada, speaks fluent Spanish, so his blog is worth watching for reports on this disease—and many others too.)