A transmission electron micrograph of Zika virus (black circles; Image: CDC) Zika virus, a 40-nm sized capsule of about a dozen genes, is on the move. The recent flare-up of cases in Brazil has diffused across borders into Colombia and Venezuela, Mexico and Puerto Rico. And while the full extent of the virus’ spread is difficult to predict, most experts agree on one thing: it will soon gain a foothold in the United States. The South, where the requisite carrier mosquitos (two species of the Aedes genus) live, is particularly susceptible, but if the virus can evolve to hitch a ride within other mosquito species, the potential footprint expands dramatically. Last week, a panel of experts gathered at Harvard University to discuss the state of the pandemic, to recap what is known and preview the wide scope of possible scenarios. Perhaps most remarkably, biologists have little mechanistic knowledge of how Zika wreaks havoc on human hosts. Potential links with microcephaly have been widely reported, and there may also be a connection with Guillain-Barre syndrome, but proof of these linkages remains elusive. And while these prognoses are bleak, they’re also atypical. According to Michael VanRooyen, Director of the Harvard Humanitarian Initiative, the infection is “mostly asymptomatic; four out of five people who have it don’t know it.” Helen Branswell, a journalist at STAT who has covered the infectious disease beat for years, agrees: “for most people it's inconsequential,” she said, and the virus tends to burn itself out. “In some places that have had it,” Branswell notes, "there are big spikes in incidence, and then it disappears." Nonetheless, the putative link to severely debilitating conditions will lead millions of people to seek medical tests - tests that, for the moment, are painfully slow and dangerously inconclusive. Current options include a PCR-based genetic test, or an antibody analysis that would demonstrate the body’s molecular recognition of the Zika invader. Both are maddeningly unspecific. As VanRooyen explains, “testing is not easy - if you test positive for it, you may have the disease. If you test negative, you still may have the disease.” For pregnant women, the notion of getting serial ultrasounds to monitor fetus development is a costly option with uncertain medical benefits. Enter the silver bullet: a vaccine for Zika. No so fast, cautions Branswell, citing the absence of a model system within which to conduct biologically relevant tests. “To test if a vaccine would actually be protective in people, there’s no clear way to do that,” she explains. “And any company that says it has a vaccine, what they really have is an experimental vaccine that might be a product someday, but it’s not happening any time soon.” Such a hypothetical product would likely be aimed at pre-adolescent girls and pregnant women, “and the safety bar for vaccines targeted toward those populations is very high,” says Branswell. VanRooyen is even more pessimistic about the prospect: “it’s going to be a while, and the economics are such that a vaccine may never be developed." Last week, the World Health Organization published a six-month plan to coordinate a multi-national response to the pandemic with an estimated price tag of $56 million. The White House has also called for a $1.8 billion emergency fund to promote prevention and detection efforts both domestically and abroad. To Sheila Burke, a Research Fellow at Harvard’s Center for Social Policy, these sorts of commitments signal a promising change compared with past pandemic threats, "contrasting with the fragmented and delayed response to Ebola." So while much about Zika’s future path remains murky, health organizations are preparing for a global fight.