The harder scientists work to unravel the mystery of the Zika virus, the more unanswered questions they seem to generate.

In a review one month after the World Health Organization declared the Zika outbreak to be a global public health emergency, WHO Director-General Margaret Chan admitted Tuesday that “scientific gaps” still plague the global health community's understanding of the full dangers of the virus. Nevertheless, she said, “Strong public health actions should not wait for definitive scientific proof.” So far, more than 50 countries and territories have reported local Zika transmission, and the WHO is asking for $56 million to combat the outbreak.

Scientists have more evidence to back up their strong — but still unproven — suspicions that Zika infection during pregnancy causes microcephaly in babies, meaning they are born with an abnormally small head. Scientists have found the virus in the brain tissue as well as the blood and other fluids of fetuses that had been miscarried, stillborn or aborted, for instance, while a spike in cases of microcephaly in Brazil has been associated with a current Zika outbreak there. Scientists have also uncovered evidence that the virus causes other birth defects besides microcephaly, from fetal growth retardation to injuries to the central nervous system, Chan told reporters during a conference call.

Increasing numbers of reports suggest the Zika virus is connected with Guillain-Barré syndrome, but that link remains similarly unproven. And while the virus has been understood as a mosquito-borne disease, authorities have also begun reporting more widely cases of the virus being transmitted sexually — yet another area that WHO officials said merited further research and study.

All the existing evidence points in the same direction: that there is a causal relationship between Zika and all the symptoms that have so far coincided with it — microcephaly and Guillain-Barré syndrome — but there is still not enough evidence to establish those links definitively, David Heymann, chairman of the WHO's emergency committee on Zika, said Tuesday.

To help close the scientific knowledge gap, the WHO has called for prioritizing the development of diagnostics for the Zika virus, which are vital to help countries gather accurate information that can inform both scientific research and their response to the virus. Current diagnostic tests are limited in that they are most accurate only when a patient has an active infection. Tests to determine whether someone had the virus in the past are less reliable. 

But it's not just the science that seems to lack answers. The WHO has also issued travel guidelines, suggesting that women who are pregnant should "consider delaying travel to any area where locally acquired Zika infection is occurring," but it has stopped short of naming countries or urging women in countries with local transmission to avoid getting pregnant. Chan said Tuesday that the WHO's job was not to make recommendations for specific countries, but instead make sure women had access to voluntary contraception, all within their own country's laws.

As the search for answers and solutions continues, the World Health Organization and scientists seem certain of one prospect. The number of cases will continue to grow, Chan said, and the virus will spread farther throughout the world.