“She will not walk,” a doctor told Ana Carolina Caceres’s mother on the day she was born. “She will not talk and, over time, she will enter a vegetative state until she dies,”

But the doctor was wrong, Caceres wrote in an essay for the BBC. The Brazilian journalist has lived with microcephaly, the often-devastating birth defect linked to the Zika virus, for 24 years. She does more than walk and talk; she attended university, she wrote a book, she keeps a blog. And although she endured breathing problems, seizures and a series of intensive medical interventions — five operations, 12 years of taking medication — she resents the idea that she, and other children like her, are “damaged.”

“Microcephaly is a box of surprises,” Caceres wrote. “You may suffer from serious problems or you may not.”

Yet Caceres’s experience is only one look at life with microcephaly. People born with the neurological condition exist on a spectrum. On one end are those for whom the characteristic unusually small head is largely a superficial concern. Then there are people like Caceres, who suffer some health problems but are otherwise able to live regular lives. About 15 percent of people with microcephaly have no intellectual disabilities associated with it.

But these types of cases are the minority in the current outbreak. Officials in Brazil and other countries hit by the Zika virus say that the cases of microcephaly they’re seeing are on the other, more severe end of the spectrum: cases of infants who are likely to live with profound intellectual disabilities, serious speech and motor problems and other health complications that require constant care. In Brazil, there have been at least a dozen confirmed deaths of babies born with the condition.

It’s still not completely certain that Zika virus is responsible for these cases — in declaring a global public health emergency, the World Health Organization said that a link is still only “strongly suspected.” It’s also not clear how the virus might disrupt the development of the head and brain during pregnancy, though a 2014 study of another virus tied to microcephaly found that it worked by attacking stem cells during early brain development.

But the burden of microcephaly on mothers and children meant that the WHO couldn’t afford to wait, Director-General Margaret Chan said.

To Marilia Lima, the mother of an almost three-month-old with microcephaly, the consequences are already apparent. Lima’s son Arthur was born with problems in his eyes, hips, and arm and leg bones; his head is much too small. He’s fretful and he struggles to suckle. Every time Lima goes to sleep, she worries she’ll wake up to find her son dead. “We are living with this terrible thing,” Lima told NPR. “But I’m at the point where I can’t think. I just have to act.”

At Oswaldo Cruz Hospital in Recife, Brazil, which Time described as the “ground zero” for concern about the Zika outbreak, doctors say they are seeing 10 infants with microcephaly a day. Since microcephaly is a condition, rather than a single disease, the babies show a range of problems: some can’t see, others have tantrums or seizures. And doctors can’t predict how those problems will play out later on in life.

“They ask me, ‘How will my baby look in one or two years?’ It’s difficult because the damage to the brain is severe, but we can’t make a prediction of the baby in the future,” Vanessa van der Linden, a neurologist in Recife, told Time. “I tell the mother, ‘We don’t know, we need to follow them, and we need to help them with the problems as they appear.’ ”

Before the current Zika outbreak, microcephaly had been tied to mothers’ malnutrition; infections like rubella, toxoplasmosis and cytomegalovirus during pregnancy; and exposure to toxins like alcohol, drugs and certain chemicals while in the womb, as well as genetics, the CDC said.

It was also been seen in a number of children whose mothers had been pregnant on the day of the atomic bombing of Hiroshima and Nagasaki.

Just as there is no specific cause of microcephaly, doctors can’t offer any one treatment. Though the condition is not new — there are reports from the 15th century about a French court jester who had the condition — our understanding of it is still vague. Treatments are symptomatic: speech therapy, medication to prevent seizures, physical therapy. There is no cure, no way to restore a child’s skull — and the brain inside it — to the right shape and size.

It is a sad and frightening prospect for parents hoping to have a healthy, independent child. But life with microcephaly does go on. Erika Martinez of Texas, whose daughter Erilyn turned 7 on Saturday, spoke to the BBC about raising a child with the condition. “She is developmentally delayed … she’s mentally like a 3-year-old,” Martinez said. “But she’s definitely amazing. [Parents] probably think it’s the end of the world. … But they’re not alone.”