Could factory farming cause the next pandemic?

The objection

“Pandemics are bad luck from wet markets or bats, nothing to do with the chicken in my fridge.”

The answer

The objection is right that wild animals are the commonest source of new human diseases [1]. What it leaves out is the farm in between. A 2020 report from the UN Environment Programme and the International Livestock Research Institute names rising demand for animal protein and unsustainable agricultural intensification among the seven human-made drivers most likely behind new diseases, because intensive livestock farming keeps large numbers of genetically similar animals close together, and uniform populations like these are more vulnerable to infection than varied ones [1]. The report’s example is factory pig farming, which helped swine flu spread because the animals had no room to keep apart [1]. It flags weak biosecurity on livestock farms near cities in poorer countries too [1].

Where do new human diseases come from?

Most new human diseases come from animals, and most of those from wild ones. The UN report (2020) estimates that some 75% of new and emerging human infectious diseases jump to people from other animals [1]. Jones and colleagues (2008) counted something slightly different, 335 emergence events between 1940 and 2004 rather than diseases, and found 60.3% of them were zoonoses, 71.8% of those from wildlife [2]. Farmed animals can be the original source, the route to people or the amplifier [1]. A 2019 review linked farming to more than half of the zoonotic diseases to emerge in people since 1940 [3], though that counts dams and irrigation schemes as well as factory farms [1].

Have farmed animals passed outbreaks to people before?

Farmed pigs were the bridge to people in two serious outbreaks, the Nipah virus outbreak in Malaysia in 1998 and 1999, and the 2009 flu pandemic. Nipah came from fruit bats [1]. Pulliam and colleagues (2012) describe where it began, a commercial farm of about 30,000 pigs with mango trees beside the sties, where the first known human cases, diagnosed only in hindsight, fell ill in early 1997 [4]. Repeated introductions into its high-turnover herd let the virus persist, and it probably spread to other farms through pig sales [4]. The outbreak was first noticed in late September 1998, and by mid-June 1999 Malaysia had reported more than 265 cases of encephalitis in people in close contact with pigs, 105 of them fatal (Chua et al., 2000) [5]. At least a million pigs were culled to control it [1].

Mena and colleagues (2016) sequenced flu viruses from pigs on large commercial farms in Mexico and concluded that the 2009 pandemic virus evolved in pigs in central Mexico, where similar viruses had circulated for more than a decade, mixing strains brought in by the global trade in live pigs [6]. The US Centers for Disease Control and Prevention puts the pandemic’s death toll in its first year at 151,700 to 575,400 [7], 80% of them in people under 65, from a 2012 modelling study [8].

Could bird flu become the next pandemic?

Nobody knows yet: the World Health Organization said in January 2026 that it is unknown whether today’s bird and swine flu viruses will cause a pandemic, and none has shown sustained spread between people [9]. The UN report (2020) gives bird flu as an example of a virus that can turn into a form able to infect people in bio-insecure industrial and intensive farms: the highly pathogenic kind, a disease of domestic poultry, evolves from milder viruses that wild birds carry without harm [1]. True human pandemic flu viruses, it adds, evolve by mixing in domestic animals, usually pigs and poultry, and with human flu [1].

H5N1 first infected people in Hong Kong in 1997, in cases traced to domestic and wild waterfowl in Guangdong, China, and after it re-emerged in 2002 more than 100 million chickens and ducks in Asia died of it or were culled [1]. It has since spread to marine mammals and other wild mammals, and to dairy cattle in the US [9], where the Department of Agriculture first confirmed it on a dairy farm in March 2024 and, as of September 2026, says it probably moves between farms with cattle, people, vehicles and equipment [10]. By October 2026 the US Centers for Disease Control and Prevention had counted 72 human cases of H5 bird flu in the country since February 2024, and it rates the risk to the public as low [11].

What do farm antibiotics have to do with pandemics?

Farm antibiotics feed a slower threat than a viral pandemic, the rise of drug-resistant bacteria. Animals consume about three times the antimicrobial drugs people do, as Van Boeckel and colleagues noted in 2019 [12]. Unlike in people, they wrote in 2017, farm use is mainly for growth promotion and mass prophylaxis, the dosing of whole herds and flocks to prevent disease, often in feed and at low doses that favour resistance, while in Europe regulation has been the main tool for limiting it [13]. They point to particularly strong evidence linking resistance in animals and people for quinolone-resistant Campylobacter and Salmonella [13]. Drug-resistant bacterial infections were directly responsible for an estimated 1.14 million deaths in 2021 (GBD 2021 Antimicrobial Resistance Collaborators, 2024) [14], but resistance genes reach people by complex routes, and the 2017 authors note that cutting farm use alone will not contain resistance in humans [13]. Their own analysis found that limiting meat to 40g a person a day worldwide, about one fast-food burger, could cut antimicrobial use in farmed animals by 66% by 2030 against business as usual [13].