There are many uncertainties about Zika, including whether it is directly responsible for the misshapen skulls of babies born to infected mothers. For example, there are four types of dengue virus, and the severity of infection can range from mild aches and pains all the way to an Ebola-like hemorrhagic death. For years, scientists tried to pinpoint which strain caused which symptoms, but it is now known that the worst outcomes arise when an individual is infected sequentially with different strains, triggering his immune system into a massive overreaction. With this, and other complicated examples of mosquito-carried viral diseases in mind, the WHO is very cautiously referring to an “association” between microcephaly and Zika — not yet cause and effect. Seriously complicating matters is the Zika blood test: It can only detect viruses in human blood during the first five days of infection, but average Zika incubation time is four to five days, meaning that by the time individuals seek medical help the virus is already undetectable with current diagnostics. A longer-term assay, which tests for the presence of antibodies to Zika, is so weak that it cross-reacts with dengue and chikungunya, rendering it useless in the context of concurrent spread of all three viruses.

But Zika has been circulating in South America since last April or May, and explosive spread in Brazil has been underway since October. The correlation (if not causation) between Zika, pregnancy, and microcephaly has been noted by Brazilian health authorities since November, as have a worrying number of cases of paralytic Guillain-Barré syndrome. Yet the WHO has done little publicly throughout November and December, save releasing a few press statements. This week, the New York Times and Newsweek, among others, asked whether the WHO was asleep at the wheel.

My conclusion is that public health leaders and politicians had better brace for a very long haul on Zika. The virus will hide, infecting a range of insects, perhaps monkeys, even birds. And it will return in seasonal cycles, as have other mosquito-carried viruses, such as yellow fever, West Nile virus, chikungunya, and dengue. Because so many “foreign” viruses carried by mosquitoes are now spreading across the Western Hemisphere at the same time, there will be misdiagnosis, mystery, and perhaps acute illnesses due to co-infections. Until we have an effective vaccine and have executed mammoth immunization campaigns in all of the nations of the Americas, Zika will haunt us, sicken some of us, and endanger our babies.