Athletes and tourists converging on Brazil this week are crowding into a country where rapid environmental change and natural weather fluctuations nurtured a viral epidemic that has gone global.

The Zika virus has exploded throughout South America, up through Mexico and Puerto Rico and into Florida, but the conditions it needed to fester in northern Brazil were rooted in urbanization and poverty. The initial Brazilian outbreak appears to have been aided by a drought driven by El Niño, and by higher temperatures caused by longer-term weather cycles and by rising levels of greenhouse gas pollution.

This combination of human and natural forces is emerging as the possible incubator of a disease that’s painfully elusive to detect, despite its cruel effects on unborn children.

The U.S. Centers for Disease Control and Prevention issued an unprecedented domestic travel advisory this week, warning pregnant women to avoid a Miami neighborhood where more than a dozen Zika cases were confirmed.

The warning came six months after the World Health Organization declared a public health emergency “of international concern” in Brazil, where the opening ceremony for the Summer Olympics is scheduled for Friday.

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While the effects of El Niño and other weather cycles are beyond the control of humans, the recent spread of the disease into the U.S. is a savage reminder of the heavy toll that humans are taking on their planet — and of the potential for those changes to bite back