The way we think of fire informs how we manage it. For a long time we considered it physics, but perhaps fire is a biological phenomenon, like a virus. By Stephen J. Pyne, Arizona State University What is fire? Usually, it’s defined as the physical chemistry of combustion. The definition matters because it prescribes how we think about fire and what we do with it. If fire is a chemical reaction shaped by its physical surroundings, then we can deconstruct it into its constituent parts. Fuel plus oxygen plus spark equals fire. We can harness it, put it into machines, and contain it by physical countermeasures. Another perspective might be that fire results from life – biological processes create the oxygen and furnish the fuel. Fire takes apart what plants put together through photosynthesis. Fire’s chemistry is therefore a biochemistry. Fire depends on the living world to flourish, and facilitates a wide gamut of ecological effects. Perhaps fire is less like a Bunsen burner than it is a virus. Not alive, but dependent on life, it spreads as a contagion of combustion. Often thought of as a natural disaster, perhaps fire more emulates COVID-19 than a hurricane. If so, it responds to biological, not just physical, conditions. The fire-as-biology analogy is surprisingly robust. When ancestors of modern humans captured fire, it more resembled domestication than tool-making. Tamed fire is more a sheepdog than a torch. The allusion seems strange because we are so accustomed to imagining fire in strictly physical terms and because, while fire radiates metaphors, it rarely receives them. Fire is fire, elemental: other phenomena are compared to it, not it to others. A plague spreads like wildfire. We don’t hear that wildfire spreads like a plague. Where urban spaces meet the bush – the built environment most vulnerable to burning – protective strategies are similar for wildfire as for COVID. Clearing combustibles around structures is like practicing social distancing. Sealing houses against embers and direct contact is like wearing masks against aerosols and washing hands. Building firebreaks against spread – isolating the infected. Enough buildings shielded to limit damage from the unprotected ones achieves herd immunity. Fire plagues result from breakdowns in how people and the natural world interact. They emerge, like so many new viruses, from unbalanced environments, when natural reservoirs intersect with human meddling. The flames of wildfires are not so much wild as feral. Thinking about fire in physical terms informs how to halt a fire that is blowing-and-going, but it doesn’t explain how to manage fire on a long-term landscape scale. A strategy for living with fire would mend the ecological fractures on which fire feeds and substitute controlled fire for wild. When outbreaks occur, a biological strategy for fire might respond not only with physical counterfire but with analogues of public health measures like vaccines, quarantines, and disease-specific treatments. Plenty of burning happens in the undeveloped world, and massive land-converting fires are a characteristic of developing countries (think Brazil and Indonesia). But uncontrolled conflagrations are a pathology of the developed world. Rich countries who are most committed to an understanding of fire that deconstructs it into simple combustion and puts it in machines are the ones that have the poorest fire ecology, experience the worst build-up of fuels, and contribute the most to climate change. Earth first experienced fire when plants began to colonise continents. Fire and early life on Earth evolved together – call it first nature. Then human ancestors acquired the ability to systematically manipulate fire. Early people grew bigger brains and smaller digestive systems because they learned to cook their food. With this second fire as a catalyst, humans created a second nature. But it was one with internal checks and balances since burning was contained within living landscapes. Ever looking for more stuff to burn, people turned to lithic landscapes – once living, now fossilised plants – coal and oil. These burned in special combustion chambers, and they lacked ecological barriers and buffers; they could burn day and night, winter and summer, through wet and dry. The advent of third fire resulted from the dominance of the physical model in science and practice. And this unbounded combustion is unhinging the climate, acidifying oceans, and forcing living landscapes to reorganise. This third fire is remaking the planet, informed by fossil-fueled societies that are sublimating fire into electricity, replacing animals with machines, and rewiring geochemical cycles with plastics and petrochemicals. The third fire challenged the first two.