The vast majority of viruses are not man-made. Viruses have existed for billions of years, evolving alongside every form of life on Earth, and the roughly 10,000 known viral species represent a tiny fraction of the millions estimated to exist in nature. That said, scientists have developed the ability to synthesize and modify viruses in laboratory settings, which is where the question gets more nuanced.
How Viruses Emerge Naturally
Most new viruses that infect humans originate in animals, a process called zoonotic spillover. This isn’t a single event but a chain of conditions that have to align. First, a virus needs to be circulating at high enough levels in an animal population, whether bats, birds, rodents, or livestock. The virus then has to leave that host, typically through saliva, feces, blood, or respiratory droplets. For avian flu, the key factor is viral load in the intestinal and respiratory tracts of poultry. For rabies, it’s the concentration of virus in a carnivore’s salivary glands.
Once outside the animal, the virus must survive long enough to reach a person, either through direct contact, contaminated water, airborne particles, or an insect bite. Then comes the hardest part: the virus has to successfully enter human cells, hijack their machinery to replicate, and overcome the immune system’s defenses. Even when a virus can get inside a human cell, multiple barriers can prevent it from spreading to other cells and establishing a real infection. Most animal viruses never clear all these hurdles, which is why spillover events are relatively rare despite constant human-animal contact.
This process has been responsible for HIV (which crossed from chimpanzees), Ebola (likely from bats), SARS (from bats through civets), MERS (from bats through camels), and countless influenza strains that jump from birds or pigs. Each of these viruses carries clear evolutionary signatures, genetic evidence of gradual mutation and adaptation that unfolds over decades or centuries.
Scientists Can Build Viruses From Scratch
In July 2002, a team led by virologist Eckard Wimmer announced they had chemically synthesized poliovirus in a lab, assembling its entire genetic code (about 7,500 genetic letters) from mail-ordered DNA fragments without using any natural virus as a physical template. They stitched together short, overlapping pieces of DNA, converted the result into the type of genetic material poliovirus actually uses, then mixed it with a cell extract from human cells. The viral proteins assembled themselves into infectious particles.
The team deliberately inserted 27 small genetic changes across the synthetic virus’s code to serve as “watermarks,” proving it was lab-made. When grown in cells, this synthetic version behaved identically to the natural virus.
Since then, researchers have synthesized several other viruses. The 1918 influenza strain, which killed tens of millions of people, was reconstructed from its published genetic sequence to study what made it so deadly. Scientists also built an infectious version of the bat coronavirus closely related to SARS (about 29,700 genetic letters, one of the largest synthetic viral genomes created) to study how coronaviruses jump between species. Researchers even resurrected an ancient retrovirus embedded in the human genome, piecing together a working virus from degraded genetic fragments scattered across our own DNA.
These projects are driven by specific scientific goals: understanding what makes a virus dangerous, developing vaccines, and preparing for future outbreaks. But they demonstrate that the technical capability to build viruses exists and is advancing.
How Scientists Spot Lab-Made Viruses
When researchers engineer a virus, the process typically leaves detectable traces in its genetic code. The tools used to cut, paste, and assemble DNA leave behind short sequences that don’t appear in nature. Computational tools can scan a virus’s genome for these synthetic “signatures,” short stretches of DNA (as few as 23 genetic letters long) that match known laboratory tools but don’t show up in any natural viral, bacterial, or plasmid sequence.
Researchers have identified a set of 364 such signatures that cover nearly all commonly used lab DNA tools, creating a kind of forensic database. This redundancy makes detection robust: even if a virus is heavily modified, the odds of erasing every trace are low. That said, newer genetic engineering techniques are becoming more precise, and the field of detecting manipulation is in an ongoing arms race with the techniques used to perform it.
A naturally evolved virus, by contrast, shows a continuous record of gradual mutations accumulated over time. Its genetic code fits neatly into an evolutionary family tree with related viruses found in animal populations. Lab-made viruses tend to lack this evolutionary history or contain combinations of genetic features that don’t match any plausible natural lineage.
Gain-of-Function Research
Between “fully natural” and “built from scratch” lies a gray area: research that takes an existing natural virus and modifies it to study specific properties. Gain-of-function research, broadly defined, involves any experiment that gives a pathogen new or enhanced abilities. In practice, the term is most often applied to experiments that make a virus more transmissible or more dangerous to help scientists understand what mutations to watch for in nature.
The most well-known example involved H5N1 bird flu. Researchers repeatedly passed the virus through ferrets (the best animal model for human flu transmission) until it acquired the ability to spread through the air between animals, something the wild virus couldn’t do. The goal was to identify the specific mutations responsible, so public health officials could monitor circulating bird flu strains for those same changes. This type of work is intensely controversial because of the risk that an enhanced pathogen could accidentally escape a lab.
The U.S. government paused federal funding for certain gain-of-function experiments in 2014 and later established a review framework for proposed studies. Even among virologists, the term remains contentious. As one leading researcher noted, “gain of function” is a standard genetics term being applied in an unsatisfying way to a very specific subset of high-risk virology work.
Viruses Modified for Medicine
Not all lab-modified viruses are created to study disease. Many are deliberately engineered to prevent it. Several COVID-19 vaccines, for instance, used a modified adenovirus (a common cold virus) as a delivery vehicle. Scientists stripped the adenovirus of its ability to replicate by deleting key genes, then inserted genetic instructions for the SARS-CoV-2 spike protein. The result was a harmless virus shell that could enter your cells and teach your immune system to recognize COVID-19 without causing infection.
This approach, called reverse genetics, is also used to rapidly produce updated flu vaccines. Researchers can take the genetic sequences of newly emerged flu strains and use them to generate vaccine candidates in cell cultures, bypassing the slower traditional method of growing viruses in eggs. These are “man-made” viruses in a literal sense, but they’re specifically designed to be unable to cause disease.
The HIV Conspiracy Theory
One of the most persistent claims about man-made viruses involves HIV. Theories that the virus was created by the CIA or the U.S. government as a bioweapon have circulated since the 1980s. There is no evidence supporting these claims. Genetic analysis has firmly established that HIV-1 evolved from a closely related virus found in chimpanzees, and the most likely route of transmission to humans was through exposure to infected blood during the hunting or preparation of bushmeat. Researchers have even synthesized the chimpanzee virus in the lab specifically to study this evolutionary jump, confirming the genetic relationship between the two viruses.
The COVID-19 Origin Question
The origin of SARS-CoV-2 remains the most prominent active debate about whether a virus could be lab-derived. As of mid-2025, the WHO’s Scientific Advisory Group concluded that the weight of available evidence suggests the virus reached humans through zoonotic spillover, either directly from bats or through an intermediate animal host. However, the group also stated that all hypotheses must remain on the table, including a potential lab leak.
A key reason the question remains unresolved is that China has not shared critical data that WHO and independent scientists have repeatedly requested: detailed genetic sequences from early COVID-19 patients, information about animals sold at Wuhan markets before the outbreak, and data on research activities and biosafety conditions at laboratories in Wuhan. Without this information, a full evaluation of all hypotheses isn’t possible.
What the scientific community has not found is evidence that SARS-CoV-2 was deliberately engineered. The debate is between natural spillover (the virus jumped from animals to humans in a market or similar setting) and a research-related incident (the virus may have been collected from nature and accidentally released from a lab studying bat coronaviruses). These are very different scenarios from the idea that someone intentionally designed the virus.
International Rules on Viral Research
The Biological and Toxin Weapons Convention, in force since 1975, prohibits member states from developing, producing, or stockpiling biological agents that have no justification for peaceful, protective, or medical purposes. Over 180 countries are parties to this treaty. It requires nations to control biological research activities within their borders, and review conferences are held every five years to address emerging threats and technologies.
At the laboratory level, work with dangerous pathogens is restricted to high-containment facilities with strict safety protocols. Research involving the most dangerous known pathogens, or experiments that could make a pathogen more dangerous, requires the highest biosafety levels and, in many countries, government review before the work can begin. These controls aren’t perfect, and accidental lab releases have occurred throughout history with various pathogens, but the framework exists specifically because the scientific community recognizes the risks that come with the ability to manipulate viruses.