The recent hantavirus outbreak has once again raised a question that scientists have been warning about for years: are zoonotic spillovers — the transmission of pathogens from animals to humans — becoming less of an exception and more of an inevitability? If Covid was the defining reminder of how devastating an animal-borne virus can become, hantavirus is the latest signal that the ecological conditions enabling such spillovers are a risk.

As forests are cleared, cities expand deeper into wildlife habitats, industrial farming intensifies, and climate change reshapes the movement of species and disease-carrying vectors, such outbreaks can become more frequent. Dr Gagandeep Kang, Director-Enterics, Diagnostics, Genomics and Epidemiology, Global Health, Gates Foundation, tells The Indian Express how humanity is creating precisely the conditions in which new infectious threats are more likely to emerge.

As human expansion increasingly overlaps with wildlife habitats, will zoonotic spillovers become more frequent rather than exceptional? What does current evidence suggest?

The evidence strongly suggests we are. Spillover events are not random; they are driven by specific conditions. Deforestation, agricultural encroachment and unplanned urban growth mean that wildlife reservoirs are no longer at a distance from dense human populations. At the same time, global travel and trade mean that once a pathogen crosses the species barrier, the window for containment is becoming very, very short.

The data also show that the list of pathogens with spillover potential is far longer than the ones that have made headlines so far, including filoviruses, influenza and coronaviruses. Spillovers have always occurred — what has changed is the frequency of the conditions that amplify them into outbreaks and the speed with which local events become global ones.

I think the two main categories to consider are industrial livestock farming and deforestation-driven habitat fragmentation as systematically underregulated and poorly tracked drivers of human-animal interaction. Industrial farming creates enormous concentrations of genetically similar animals, which can amplify pathogens with pandemic potential. Influenza is the clearest example, but it is far from the only one, and can also happen in backyard poultry farming, as we have seen in Asia. The danger with the industrial production of animals is the potential for widespread dissemination of those products in the context of limited bio-surveillance for pathogens.

Deforestation is different— it creates contact between human communities and reservoir hosts that previously had little reason to intersect. Nipah in India, Malaysia and Bangladesh, Ebola in Central Africa follow a recognisable pattern of habitat loss that pushes bats and rodents into human-modified landscapes.

Bluntly, no — not in the way the question implies. We have made real investments in surveillance infrastructure since SARS (Severe Acute Respiratory Syndrome), and national reporting has genuine value. But we are still responding to outbreaks, not predicting them.

One part of the reason is the surveillance system, which for emergencies is still mainly human clinical systems. Veterinary surveillance, wildlife monitoring, and environmental sampling — the earliest points in the spillover chain —are underfunded and fragmented, though efforts have been initiated in some countries, including India, for One Health surveillance. It is important to recognise that a pathogen can circulate in an animal reservoir for years before it generates a human case visible enough to trigger a formal alert.

The most clearly documented mechanism is the range expansion of arthropod vectors. Aedes aegypti and Aedes albopictus are establishing themselves at higher altitudes and latitudes than previously recorded. Tick distributions are shifting. Anopheline mosquito populations are appearing at elevations in East Africa where they were historically absent.

The picture for directly transmitted zoonoses is more complex but no less concerning. Changes in rainfall patterns, temperature and land productivity are altering animal behaviour and migration routes, creating new contact points with human populations. Bat roost stability is disrupted by climate variability, potentially increasing virus shedding. Rodent population dynamics shift with agricultural patterns themselves responding to climate pressure. These are indirect pathways, but they operate simultaneously and interact with the habitat disruption drivers I mentioned earlier.

What worries me particularly is the intersection of climate change with fragile health systems. The populations most exposed to climate-driven shifts in disease geography are frequently those with the least capacity to detect and respond to novel outbreaks. That mismatch between burden and capacity is not new in global health, but climate change is making it clearer.

What practical changes are most urgently needed to reduce the risk of the next major zoonotic outbreak?

At the surveillance level, the most urgent investment is in integrated One Health monitoring that spans the animal-human-environment interface with standardised protocols and real-time data sharing. This means adequately funding veterinary and wildlife surveillance systems, not just human clinical networks, and ensuring that data from those systems actually flows into national and international response mechanisms without delays.

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And at the global governance level, the International Health Regulations need strengthened compliance mechanisms and a serious rethinking of the incentive structures around early reporting. The Pandemic Accord negotiations are an opportunity to address this, and I hope those discussions move forward.

Should zoonotic diseases now be considered one of the defining public health challenges of this century?

Yes. Covid was not a black swan. It was the kind of event that infectious disease scientists and pandemic preparedness advocates had been explicitly warning about for years. The same is true of the preparedness gaps it exposed — insufficient manufacturing capacity, inequitable access to countermeasures, surveillance systems that struggled to keep pace, and international governance mechanisms that bent under political pressure. None of this was unforeseen. What was lacking was the institutional will to act on the foresight that existed.

Zoonotic diseases are a defining challenge because of the convergence of drivers — ecological disruption, climate change, globalisation, and antimicrobial resistance in animal reservoirs. This means that this is a threat that will continue to evolve, making preparedness an urgent and necessary policy for global and public health.