Four facts about the Zika virus have generated great fear. First, it has spread quickly to wide areas. Second, those infected with the disease often show no symptoms. Third, it has been linked to a surge in cases of microcephaly among new-born children. And finally, fourth, there is no vaccine. That combination is a recipe for concern and near panic in Latin America, where the Zika virus is widespread. Health authorities must move quickly to reassure their troubled publics, spearhead efforts to get a vaccine out and push governments to do more to eliminate the source of the problem. Sadly, this is a likely rehearsal for more such outbreaks.

The Zika virus was discovered in Uganda in 1947, and was subsequently detected in several African and Asian countries. Medical professionals paid little attention to it until an outbreak in 2013 infected about 11 percent of the population of French Polynesia. It is believed that participants at two international sporting events held in Brazil in 2014 introduced the disease to that country; it is reckoned that as many as 1.5 million people there were infected within a year. Once it gained a foothold in Brazil it quickly spread throughout Latin America and the Caribbean. The World Health Organization believes Zika is present in 23 countries and has infected as many as 4 million people.

Its spread originally prompted little concern as 80 percent of those infected displayed no symptoms, and when they did surface, the indicators were mild: a light fever, skin rash, conjunctivitis, muscle and joint pain, and fatigue that would last from two days to a week. Those symptoms were similar to those of dengue or chikungunya, which are transmitted by the same vector — female mosquitoes.

Last October, however, doctors in Brazil discovered an alarming link between an outbreak of Zika and a growing number of babies — currently around 3,700 — born with microcephaly (abnormally small heads), brain defects and impaired vision. While the nature of the link is not yet clear — doctors do not know how the virus causes the birth defects — the correlation is strong enough for health officials to caution women in areas that host the virus to avoid getting pregnant and for pregnant women to avoid visiting infected regions.

The WHO has moved quickly to respond to the disease and the U.S. Centers for Disease Control and Prevention have established an emergency operations center to be ready for any reported cases. While virtually all cases have appeared in the Americas, other parts of the world must be prepared as well: Visitors to infected regions are potential carriers and some cases have already been reported among tourists returning home. As Brazil will be hosting the Olympics this summer, a mass infection and outbreak is possible. Olympic officials have insisted the games will be safe, but they also urged all visitors to protect themselves against getting the disease.

That is easier said than done. There is no vaccine against Zika. Research has begun but as yet there have been no clinical trials and the earliest any drug might be on the market is the end of this year. The only way to deal with the disease now is to stop it from spreading. Individuals can protect themselves with the usual tactics: mosquito repellent, long sleeves and sleeping under mosquito nets, preferably ones that are treated. Governments and health authorities should focus on eradicating the mosquitoes. Brazil has sent its army out to inspect houses and neighborhoods to identify and eliminate places where the bugs breed — pools of stagnant water. The poor, however, are likely to continue to suffer as they live in areas where sanitation is weak, water stagnates and access to repellents and nets is poor.

The spread of Zika, the newly emerging link to birth defects and the seeming inability to do much about it is disturbing. Even more troubling is that this may become the new norm in diseases. Globalization and the virtually free movement of individuals means that diseases will spread around the world with new and surprising ease. The spread of SARS and the H5N1 variant of avian flu were warnings. Climate change will alter the way that infectious diseases are transmitted, but so too will migration patterns. The mass movement of individuals from rural to urban areas will create new disease vectors, as will the increasing concentrations of individuals in cities, especially if they lack infrastructure such as water and sanitation facilities. The appearance of Ebola in West Africa in 2014 is an indication of how these people movements can facilitate the spread of disease. The movement of peoples as a result of civil unrest and war will likely compound the problem.

The WHO on Monday declared a global emergency over the explosive spread of the Zika virus, with Director-General Margaret Chan calling it an “extraordinary event and public health threat to other parts of the world.” The organization must do more than raise the alarm bells. It must be invigorating surveillance programs, helping governments stop outbreaks from occurring and working with drug companies to get in front of diseases. That is a challenge, but one that will only intensify in the years to come.

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