Recent Pandemic Viruses Jumped to Humans Without Prior Adaptation

For decades, the conventional wisdom in virology held a reassuring premise: before an animal virus could spark a human pandemic, it needed time to evolve — accumulating mutations in an intermediate host, fine-tuning its molecular machinery for human cells, and essentially "practicing" before making the lethal leap. A sweeping new study from the University of California San Diego has shattered that assumption, with profound implications for how we prepare for the next pandemic.

The Study That Changed the Equation

Published March 8, 2026, in the journal Cell, the research led by Joel Wertheim, a professor of medicine in the Division of Infectious Diseases and Global Public Health at UC San Diego School of Medicine, represents the most comprehensive evolutionary analysis of pandemic virus origins to date [1]. The team — which included researchers from Temple University, the Scripps Research Institute, and the University of Arizona — used a sophisticated phylogenetic framework to measure changes in natural selection intensity across the entire genomes of viruses responsible for some of the most devastating outbreaks in modern history [2].

The viruses under the microscope included influenza A, Ebola, Marburg, mpox, SARS-CoV (the original SARS virus), and SARS-CoV-2 (the virus that causes COVID-19). For each, the team compared mutation rates before and after the moment of spillover — the critical juncture when a virus leaps from its animal host into a human population [1].

The result was striking and consistent: across nearly every virus studied, there was no detectable evolutionary signal indicating pre-spillover adaptation. The selection pressures acting on these viruses before they entered human populations were indistinguishable from those they experienced while circulating harmlessly among their animal hosts [2].

"Rather than requiring rare, finely tuned adaptations in animals, many viruses may already possess the basic capacity to infect humans," Wertheim stated [3].

Measurable adaptive changes — the kind that make a virus better at spreading between people — typically appeared only after sustained human-to-human transmission had already begun [1].

What This Means for COVID-19

Perhaps the most politically charged finding concerns SARS-CoV-2. Since the early days of the pandemic, fierce debate has raged over whether the virus that causes COVID-19 originated from a natural zoonotic spillover — likely at the Huanan Seafood Market in Wuhan — or escaped from a laboratory, possibly after gain-of-function research [4].

The UC San Diego study adds significant weight to the natural origin hypothesis. The researchers found no evidence that SARS-CoV-2 was shaped by selection in a laboratory setting or through prolonged evolution in an intermediate host prior to its emergence [1].

"From an evolutionary perspective, we find no evidence that SARS-CoV-2 was shaped by selection in a laboratory," Wertheim said, calling the finding "another nail in the coffin for theories invoking laboratory manipulation" [3].

The study built upon earlier work by several of the same researchers. In 2022, Wertheim and co-authors Kristian G. Andersen and Michael Worobey published research suggesting the virus jumped to humans at least twice at the Wuhan market [5]. A 2025 study from the same group used bat virus evolution to argue that the wildlife trade sparked the emergence of SARS-CoV-2 in humans [6].

It is worth noting that Wertheim, Pekar, Worobey, and Andersen have disclosed that they received consulting fees and expert testimony compensation related to SARS-CoV-2 and COVID-19, a factor critics of the natural origin theory may raise [3].

The 1977 Exception: A Virus That Really Did Come From a Lab

In a twist that strengthens the study's methodology, one virus stood apart from all the others: the H1N1 influenza strain responsible for the 1977 "Russian flu" pandemic [1].

Unlike every other outbreak virus examined, the 1977 H1N1 strain displayed selection signatures consistent with laboratory propagation — specifically the kind of patterns seen in viruses grown in cell culture or passaged through laboratory animals. The strain showed remarkably limited genetic divergence from H1N1 viruses that had been isolated in the 1950s, a gap of roughly 25 years with almost no evolutionary change — a near-impossibility in nature [7].

"Our results provide new molecular evidence supporting the long-suspected idea that the H1N1 pandemic was sparked by a laboratory strain," Wertheim said [3].

This finding aligns with decades of circumstantial evidence. A 2024 risk assessment using the modified Grunow-Finke tool calculated a 62% probability that the 1977 Russian flu was of unnatural origin [8]. Nine of the ten earliest strains isolated from that outbreak exhibited "temperature sensitivity," a hallmark of viruses deliberately attenuated for vaccine development [7]. The prevailing theory holds that the virus was accidentally released during an influenza vaccine trial, possibly in response to the 1976 U.S. swine flu pandemic scare [9].

The 1977 H1N1 case serves as a critical validation: the study's phylogenetic framework can detect the signature of laboratory manipulation when it is genuinely present — and it found no such signature in SARS-CoV-2 or any of the other naturally emerging viruses [1].

The Zoonotic Threat Landscape Today

Source: WHO / KFF Global Disease Outbreaks Report 2025

Data as of Mar 10, 2026CSV

The UC San Diego findings arrive at a moment of heightened concern over zoonotic threats. The world is simultaneously grappling with outbreaks of Ebola in the Democratic Republic of the Congo, Marburg virus in Tanzania, and — most urgently — the continued global spread of H5N1 avian influenza [10].

H5N1 bird flu is now circulating widely across wild bird populations, poultry farms, and, most alarmingly, U.S. dairy cattle herds. At least 68 people in North America have fallen ill from the pathogen, and one person has died [11]. While current evidence indicates the virus has not acquired the capacity for sustained human-to-human transmission, it is showing concerning signs of mammalian adaptation [12].

When CDC scientists sequenced samples from a fatal case in Louisiana, they found mutations at two sites critical for H5N1 to replicate more efficiently in human cells [11]. The fear among virologists is that in a person co-infected with both seasonal influenza and H5N1, the viruses could exchange genetic material through reassortment — an exceedingly rare event that has nonetheless triggered the last three human influenza pandemics [11].

"It's completely out of control," warned scientists in a BBC Science Focus analysis, noting that high levels of cross-species circulation increase the odds of the virus evolving into a strain with both high transmissibility and high lethality [13].

Why "No Adaptation Needed" Is Actually Alarming

At first glance, the finding that pandemic viruses do not require special pre-adaptation might seem reassuring — one fewer step in the chain of events leading to a pandemic. In reality, it is the opposite.

If viruses can jump from animals to humans and immediately begin efficient person-to-person spread without a long evolutionary runway, then the window between initial spillover and explosive outbreak may be far narrower than previously assumed. A 2025 study from Indian researchers estimated that once a pandemic strain begins spreading in humans, the window for effective containment could be as short as 2 to 10 detections [11].

This finding fundamentally reshapes pandemic preparedness priorities. Rather than watching for viruses that are gradually adapting in intermediate hosts — a process that might offer months or years of warning — public health systems must instead focus on the far more difficult task of monitoring the vast animal-human interface where spillovers constantly occur [14].

An estimated 1.67 million as-yet-undiscovered viruses exist in mammalian and avian hosts [15]. Each one represents a potential pandemic starter that may need no evolutionary runway at all.

The Preparedness Gap

Source: KFF / WHO Global Disease Outbreaks Report

Data as of Mar 10, 2026CSV

The implications collide with a sobering reality: global pandemic preparedness may actually be declining. According to the Gavi alliance, the world may be entering 2026 less prepared than it was in the immediate aftermath of COVID-19. A 2025 Global Diagnostics Gap Assessment identified diagnostics as the weakest link in pandemic preparedness. Sharp reductions in foreign aid from high-income countries have disrupted essential health services in lower-income nations, including disease surveillance, vaccination programs, and emergency preparedness infrastructure [16].

At the same time, the ecological conditions driving spillover are intensifying. Deforestation, urban expansion, mining, intensive agriculture, and the global wildlife trade are all weakening the ecological barriers that have historically separated human and animal populations [15]. Climate change is altering the geographic ranges of disease-carrying vectors and shifting patterns of animal migration, creating new hotspots for viral emergence [16].

The WHO's Strategic Plan for Coronavirus Disease Threat Management promotes integrating surveillance for influenza, coronaviruses, and other respiratory threats into routine health systems, using multi-pathogen platforms, genomic sequencing, and wastewater monitoring [16]. At the 2026 World Economic Forum, two complementary global digital platforms were announced: the Pandemic Preparedness Engine and the Global Pathogen Analysis Platform, which combines bioinformatics and AI to connect surveillance systems across countries and sectors [17].

A New Forensic Framework

Beyond its implications for preparedness, the UC San Diego study establishes something the scientific community has lacked: a reliable genomic benchmark for distinguishing natural zoonotic spillovers from laboratory-associated releases [1].

The methodology — measuring shifts in selection intensity across entire viral genomes at key evolutionary transitions — effectively creates a molecular "fingerprint" of how a virus arrived in humans. Natural spillovers display a characteristic absence of unusual selection pressure before the jump. Laboratory-handled viruses, as demonstrated by the 1977 H1N1 case, display telltale signatures of artificial passage [2].

This framework has immediate practical applications. When the next novel virus emerges — and with 1.67 million undiscovered viruses in animal reservoirs, the question is when, not if — investigators will have a validated tool to rapidly assess whether it arose from a natural spillover event or escaped from a research facility [1]. In an era of eroding trust in public health institutions, having an objective, reproducible scientific method for answering the "where did this come from?" question could prove invaluable.

The Bottom Line

The UC San Diego study delivers a paradoxical message: the viruses that cause pandemics are simultaneously less specialized and more dangerous than we thought. They do not need an evolutionary dress rehearsal before entering the human stage. They can walk straight from the wings and begin performing.

Since 2003, zoonotic disease outbreaks have caused over 15 million deaths and $4 trillion in economic losses globally [15]. COVID-19 alone killed an estimated 7 million people by official counts, with excess mortality estimates running far higher [18]. These catastrophes were not preceded by detectable viral "warming up" in intermediate hosts.

The lesson is clear: the next pandemic will not announce itself with a slow evolutionary buildup. It will arrive suddenly, from an animal reservoir, already capable of spreading between humans. Our surveillance systems, our diagnostic infrastructure, and our public health preparedness must be designed for that reality — not for the comforting but incorrect assumption that nature will give us time to prepare.

As Wertheim and his colleagues have shown, it won't.