The Argentine province of Tucumán has never recorded so many chikungunya infections in a single outbreak. With 423 confirmed cases by the twentieth epidemiological week and a weekly rise of 33 per cent, the numbers tell a story of conditions that were ripe long before the virus arrived. The Aedes aegypti mosquito, which also transmits dengue and Zika, does not create its own breeding grounds; it simply exploits what is already there. As Giselle Rodríguez, a researcher at the local faculty of natural sciences, put it, any scrap of disorder—a discarded bottle cap, an unmaintained swimming pool—becomes a nursery for the insect. In the low-lying southeastern districts of the provincial capital, recurrent flooding from unplanned urbanisation now provides standing water for months on end. Climate change, with its intensifying rains and lengthening summers, has turned the subtropics of northern Argentina into year-round habitat for a pest that once vanished with winter.

Viewed from a global perspective, the Tucumán outbreak is merely the most vivid symptom of a wider phenomenon. Mosquitoes are the vectors for roughly 17 per cent of all infectious diseases and kill an estimated 760,000 people annually, a toll that surpasses every other animal. As temperatures rise, the geographical reach of species such as Aedes aegypti expands, bringing chikungunya, dengue, yellow fever and Zika to latitudes that were once inhospitable. The scientific debate over whether humanity could eradicate the most dangerous mosquito species collides with a more immediate reality: urban environments, particularly in the global south, lack the drainage, waste management and public hygiene needed to deny the insect a foothold. In Tucumán, doctors already report chronic joint pain and other sequelae that persist long after the acute phase of chikungunya, adding a burden of long-term morbidity to the acute caseload.

Yet mosquitoes are not the only threat making a comeback. In January the United Kingdom lost its measles elimination status after childhood vaccination rates fell below the 95 per cent threshold required to contain the virus. Outbreaks continue across the United States. The cause, in both countries, is vaccine hesitancy—a phenomenon that anti-vaccine activists have long misrepresented by portraying measles as a trivial respiratory infection. Scientists now understand that the virus’s primary target is the immune system itself: it can destroy immunological memory, leaving survivors vulnerable to infections they had already conquered, and trigger rare, fatal neurological disorders years after the initial illness. The parallels with chikungunya’s chronic phase are striking: both diseases exact a price long after the fever subsides, and both could be substantially controlled through proven public health measures.

Brazil, meanwhile, is waging a parallel battle against misinformation over influenza vaccination. More than two million doses have been administered in the current campaign, yet myths persist that the vaccine can cause the disease or is unnecessary for those who rarely fall ill. The pattern is familiar: when confidence in immunisation erodes, preventable outbreaks accumulate. Viewed from Brasília, as from London or Buenos Aires, the challenge is less technical than cultural. Health authorities must rebuild trust in the very tools that relegated measles and seasonal flu to the margins of public consciousness only a generation ago.

Forward-looking analysis suggests that the convergence of climate-driven vector expansion and vaccine scepticism will produce more frequent, overlapping epidemics. Cities from São Miguel de Tucumán to Manchester are being forced to adapt infrastructure to a warmer world, but adaptation lags behind the pace of change. Analysts in London note that the United Kingdom’s loss of measles elimination is reversible only through sustained, high-coverage vaccination; the same logic applies to chikungunya, though no vaccine is yet widely deployed. The Argentine documentary tracing the link between climate change and infectious disease ends with a warning: the mosquito is simply finding what we have left for it. The same could be said of the pathogens we choose not to immunise against.