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Tom Koch is adjunct professor of medical geography at the University of British Columbia and the author of Cartographies of Disease (2005) and Disease Maps: Epidemics on the Ground (2011).

There is nothing truly unusual about Zika virus, the latest in a long line of microbes to expand their territory and threaten our world. Suspected of causing microcephaly in newborns, that terrifying possibility is why the World Health Organization declared an otherwise fairly benign virus a health emergency.

HIV was the beginning of what has been an expanding array of infectious diseases whose worldly spread has created angst, fear and, of course, illness. Following it, in a partial list, was SARS, MERS, West Nile Virus and, more recently, Ebola. There have been others less well known.

The important question is not why this virus is spreading at this time, but why bacteria and viruses seem to be ever-more-rapidly expanding threats. Ask who is to blame and the answer is clear: We are.

Bacteria and viruses are typically homebodies comfortable in local ecologies where they achieve a nice balance among the local inhabitants: animal, insect, and human. They move when the environment changes – through deforestation or urbanization – or when humans carry them to new places. Mindlessly intelligent, when forced to, they adapt.

Zika virus, for example, was first identified in Uganda in 1947; its transmissible agent was identified in 1952. It was just another mild, unremarkable local African flavivirus transmitted by the mosquito Aedes aegypti. And there it stayed for a time. Mosquitoes are not adventurous on their own but when caught in the wheelbase of a jet or inadvertently packed with goods being shipped around the globe, they become world travellers.

From 1952 to 2007, Aedes mosquitoes with Zika virus travelled from Uganda to other parts of Africa and, from there, into India, Micronesia, Polynesia. More recently, trade and travel have brought the mosquito and the virus to Central and South America.

Viruses are evolutionary geniuses, rapidly changing when confronted with new populations or challenges. Influenza is the greatest example, evolving year by year as we try to figure out in the spring what its makeup will be when flu season starts in the autumn. If we guess right, vaccines are effective. When we guess wrong, they are not.

What is terrifying about Zika virus is its possible effect on developing human fetuses, a new microcephaly in infants not seen before. It may be a case of genetic drift, a small transformation in the virus rather than a genetic shift. Because there is no obvious evolutionary advantage, it is likely a kind of unintended consequence of the virus's evolutionary response to its new environments. That's no comfort to the pregnant mothers who have been infected, of course.

One reason previously stable, local microbes have been evolving, expanding into new areas, is that rapid global trade and travel has carried previously local disease vectors – mosquitoes, ticks, and insects – around the world. Another reason is widespread deforestation and urbanization that together change forever the old habitats where microbes once lived in stable situations. As we change our world, we change their world, too. As importantly, in our push to modernize, we create ever-larger slums of poor persons who are the perfect breeding ground for microbes seeking new hosts.

At one level, the viruses now afflicting us, like Zika, are the cost of doing business in a global and interconnected world. To stop this or the next virus will require attention first to the health of dense populations of poor people living on the edge of megacities, to migrant worker populations and, as Ebola demonstrated, those who still live in isolated rural areas.

We have known since the days of plague that travellers sometimes carry infections with them. And, too, we've understood that poverty is a perfect field for infections to expand. But we forget. It's easier to spray a pond to prevent mosquito growth than to attend to the environmental causes of infectious disease. The result is always short term, however. Viruses and mosquitoes evolve to meet our short-term solutions every time. The resulting cost, in human lives, is always greater than good public health would have been.

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