What is the Zika virus, and how afraid should we be? A scientist explains.

Three years ago, the Zika virus was nowhere to be found in the Western Hemisphere. But in 2015, Brazil suddenly found itself in the throes of an unprecedented Zika outbreak — with more than a million people infected by the mosquito-transmitted disease.

The vast majority had nothing to worry about, at worst getting a rash and flu-like symptoms. But recently, scientists have realized that Zika may pose a unique threat to pregnant women.

Those infected with Zika during pregnancy appear to be able to transmit the virus to their fetuses. In some cases, this may lead to microcephaly, a terrible congenital condition that's associated with a small head and incomplete brain development. Babies born with microcephaly have a limited life expectancy and poor brain function.

Researchers still don't fully understand the link — or the precise risk involved — but evidence is mounting that there's some sort of relationship between the virus and birth defects. The Centers for Disease Control and Prevention is now warning pregnant women, and women who are thinking of becoming pregnant, to stay out of countries in South America and the Caribbean where the virus is circulating. Zika has also reached Puerto Rico and could well spread to the US mainland this spring or summer.

So how afraid should people — particularly women — be of Zika? I spoke to Scott Weaver, the director of the Institute for Human Infections and Immunity at the University of Texas Medical Branch in Galveston. He's been studying the virus for eight years and tracking the outbreak in Brazil. Here's a transcript of our conversation, edited for length and clarity.

Julia Belluz: How afraid should people be of Zika?

A health worker fumigates to prevent dengue, chikungunya, and Zika viruses in Lima, Peru. (AP Photo/Martin Mejia)

Scott Weaver: Just this morning I saw data from Brazil on the number of cases of microcephaly in one of the main affected areas. The overall rate of microcephaly to any woman who delivers there is higher than I would have expected.

I'm a little more concerned than I was a couple of weeks ago when I was thinking this is a rare or uncommon outcome for infection during the first trimester of pregnancy. It's looking more like it might be quite common.

JB: Do you think the CDC's recommendations — that pregnant women and women of childbearing age avoid travel to countries where Zika is circulating — are overly cautious?

SW: I don’t think they are being overly cautious. If I had a daughter who was pregnant or trying to get pregnant, I would not want her to be going to Brazil now. Even in the Caribbean, the number of infected islands is growing daily. And we are always behind in the surveillance.

Some areas are not yet on the list but are likely to be soon, so I'd be cautious about traveling. You can protect yourself from mosquitoes but not 100 percent. I'd strongly urge women in those categories [who are pregnant or considering getting pregnant] to postpone their trips.

JB: For women who are considering getting pregnant soon, how long could the virus stay in their bodies?

SW: We think the maximum incubation period is 12 days. Let's say you get infected on the last day of a visit to Brazil. You might go 12 days from then until you start developing symptoms — or no symptoms but having the virus in your bloodstream that could infect the fetus. That period lasts about a week or a little longer.

That would take you out close to three weeks. We'd expect the virus would be gone from your system by 21 days. Therefore, if you got pregnant later, it's unlikely that you'd transmit the virus to your fetus.

JB: Are women in any trimester of pregnancy at risk of birth defects?

SW: In the vast majority of microcephaly cases, the timing of infection seems to mostly point to the first trimester, perhaps early in the second, rather than the third trimester. Unfortunately, it's usually difficult to diagnose microcephaly until late in the second trimester and that's only with good prenatal care and ultrasound being performed. A lot of cases in Brazil were not being diagnosed before pregnancy.

Javier Zarracina/Vox

JB: Some small studies have suggested Zika can be sexually transmitted. What does that mean for women who haven't traveled to Zika-infected countries but who have partners who have?

SW: That’s the other thing. It seems like men who get infected can have a chronic presence of the virus in their semen and transmit it later to their partner. So there's some risk they can still transmit later.

JB: The CDC advised only pregnant women who have traveled to these countries and have symptoms of Zika to get tested. But in most cases, people with Zika don't show any symptoms. So aren't the CDC's recommendations too narrow?

SW: If you started asking every pregnant woman who had visited any endemic country to get tested, you’d be talking about tens or hundreds of thousands of women coming and asking for Zika testing. We don't have the capacity to do that. There are no commercial companies offering a Zika test, so it's only the CDC and a few state health labs.

[Note: I followed up with a CDC official, Denise Jamieson, who said that most of the birth defects have been detected in women who had symptoms of Zika. She also said that there are certain risks of testing, including false positives, which caused the CDC to make more judicious recommendations.]

JB: It seems we have pretty strong evidence, both at the population level and in individual moms and babies, on the link between Zika and birth defects. But do we know yet whether there is a detection bias? Maybe more people are looking for microcephaly now, and more cases are being reported.

SW: It wouldn't surprise me if the obstetricians in Brazil are paying closer attention and maybe, in marginal cases that they wouldn't have reported it before, are reporting microcephaly now. Maybe they are measuring every head of every baby. But I don't think this can account for most of what we're seeing.

JB: What's the outlook for outbreaks here in the US?

SW: There's still a lot we don't know about this virus, how efficiently it’s transmitted. If you look at the history of dengue and chikungunya [other viruses that are also transmitted by mosquitoes] in the US, there will likely be local transmission in the southern US, probably not until later in the year. I think risk will pick up later in the spring and summer [when mosquitoes are more active].

(Javier Zarracina/Vox)

It also depends on rainfall. Typically, the rainy season starts around June or July, and that will also increase the risk.

JB: Brazil has seen millions of people infected, and there are many tragic stories of children with birth defects, likely related to the virus. What kind of numbers would you expect in the US?

SW: History does not tell us we should expect major epidemics with millions infected, like we are seeing in South America. We might expect up to thousands of people. If we do see thousands, though, we could have some pregnant women getting infected and some locally acquired microcephaly here.

JB: I've read that there's less spread of mosquito-borne viruses in the United States than in places like Brazil because more people live with screens on their houses and have air conditioning, which keeps out mosquitoes.

SW: That's right — it's mostly our lifestyle and our ability to keep mosquitoes out of our houses here that protects us. In many parts of the country, we don't have the types of mosquitoes that transmit the virus at all.

JB: Why hasn’t microcephaly been a concern in previous Zika outbreaks elsewhere, in Africa and Asia?

Javier Zarracina/Matt Moore

SW: Since microcephaly started to peak in Brazil, French Polynesian public health people went back to their data [from a 2013-'14 outbreak] and found there was an increase in incidence in some of these neurological complications in pregnancies after the outbreak.

They also detected Guillain-Barré syndrome [a very rare neurological syndrome that attacks the muscles and can cause paralysis] before the virus reached Brazil. The Yap Island outbreak was a much smaller population, and there was no detection of either of those birth complications. But you might not expect to see that when it's that small a number of people.

JB: Wouldn't immunity also be a factor — the fact that Zika is now showing up in populations that have never experienced the virus?

SW: Exactly. A year ago, everyone in the [Western Hemisphere] was naive to Zika except for a handful of people. Every woman who was pregnant nine months ago or so in Brazil was at risk of having a birth complication.

Maybe that's part of the reason we haven’t seen this in the past in Africa and Asia, where the virus has circulated for years. A lot of people there are infected earlier in life and therefore not at risk for infection during pregnancy as much as in this naive population in the Americas.

JB: What about the risk to the general population, including older people or people with compromised immune systems?

SW: There have been a few other reports of more serious Zika infections that could be related to something like being immunocompromised or elderly. I think if there are enough cases of any viral infection, you will find a few of those outliers.

But for the vast majority of people, this is a benign and self-limited illness. The main complaints are that the rash can itch a lot, and symptoms you get with a usual flu — like body aches and pains and fever. Most people have no symptoms at all.

JB: What are the biggest unanswered questions you still have about Zika?

SW: How many pregnant women are infected in the first trimester actually develop microcephaly in offspring? Can this happen in asymptomatic cases or mainly in symptomatic women? Can asymptomatically infected people infect mosquitoes as well as symptomatically infected people? Can asymptomatically infected men transmit the virus through sexual intercourse as well as symptomatically infected men?

JB: Final question: Why are some people bitten by mosquitoes more often than others?

SW: We think it has to do with some differences in the chemical composition in people’s skin, what's emitted in their odors, that have different effects on mosquitoes' host-seeking behavior and whether they will be attracted to you or somebody nearby.

But we don’t understand this completely, to the point where we can take a swab and say, "You’ll be highly attractive or less attractive to mosquitoes." Body temperature can have an impact, too. Mosquitoes are attracted from long distances by carbon dioxide, so if you have a higher metabolic rate, you're exercising a lot, you'll emit more carbon dioxide.

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