All it takes is a little bit of water and a warm breeze.
Aedes aegypti, the main mosquito species that transmits Zika virus and several other serious diseases, can breed practically anywhere. These mosquitoes will lay eggs in the stagnant water collected in bird baths or old tires, sure, but also in the few droplets gathered in a discarded candy wrapper, or on the damp surface of a fallen leaf, or in a practically-empty soda can.
This startling resilience is part of what makes Aedes aegypti so efficient a vector and, in turn, so serious a threat to humans. Zika was for decades considered to be a mild virus, mostly just a nuisance, but is now known to cause serious health problems including grave outcomes for fetuses like abnormally small heads, severe brain defects, and even death.
In Brazil, Aedes aegypti is at the center of a Zika outbreak that global-health officials estimate will cause more than 2,500 cases of microcephaly. (As of March 22, the World Health Organization said 863 cases of the birth defect had been traced back to Zika infections in Brazil.)
The mosquitoes responsible for spreading Zika are a formidable foe. Each female mosquito can lay about 1,000 eggs in her short lifetime, and she can lay eggs just about anywhere there’s a bit of moisture and warm enough temperatures. Eggs laid on dry surfaces can survive for more than a year, and will wait to hatch until they get wet. Once that happens, if it’s still too cool out for the baby mosquitoes, they can stay in the larval phase for many months more, as long as they have enough water to stay submerged.