Scientists create and modify viruses in laboratories for several important reasons: to develop vaccines, treat diseases like cancer, deliver gene therapies, fight antibiotic-resistant bacteria, and prepare for future pandemics. While the idea of deliberately making a virus sounds alarming, most laboratory-created or modified viruses are designed to be weaker than their natural counterparts, not stronger. The small fraction of research that does make pathogens more dangerous is heavily regulated and deeply controversial.
Making Weakened Viruses for Vaccines
The oldest and most common reason to create a virus in a lab is to build a vaccine. Live attenuated vaccines, which use a weakened version of the real virus, remain among the most effective vaccines ever developed. The vaccines for smallpox, polio, yellow fever, measles, mumps, and rotavirus all rely on viruses that were deliberately altered so they could still trigger a strong immune response without causing serious illness.
Scientists weaken viruses through several techniques. One classic method is serial passaging: growing the virus over and over again in animal cells until it adapts to those cells and loses its ability to thrive in human tissue. The yellow fever vaccine strain 17D, developed this way, actually enters human cells through a completely different doorway than the dangerous wild-type virus. Measles vaccine strains similarly shifted which receptors they use to get into cells, making them far less effective at infecting the tissues where natural measles does its damage.
A key change in many weakened vaccine viruses is what happens to the body’s alarm system. Wild measles and yellow fever viruses block the production of interferons, the signaling molecules your immune system uses to raise the alarm about an infection. The vaccine strains lost that ability, so your body detects and controls them quickly. Scientists now use genetic engineering to deliberately knock out these immune-evasion genes in viruses, or to disable genes the virus needs to copy its own DNA, producing reliably weakened strains for new vaccines.
Another approach is temperature sensitivity. Cold-adapted influenza vaccines carry mutations that let the virus replicate at cooler temperatures (around 25°C) found in the nose and upper airways, but not at the 37°C core body temperature deeper in the lungs. This limits the infection to a mild, superficial one that still trains the immune system.
Treating Cancer With Engineered Viruses
Some viruses naturally prefer to infect and kill cancer cells over healthy ones. Scientists have learned to exploit this by engineering “oncolytic” viruses, viruses designed specifically to destroy tumors. A modified version of the herpes simplex virus (HSV-1), known as T-VEC, completed phase III clinical trials in patients with advanced melanoma and showed significant results. It became one of the first virus-based cancer therapies to reach widespread clinical use.
The logic behind these viruses is elegant. Cancer cells often disable the same antiviral defenses that normal cells rely on. For example, reovirus specifically replicates inside cells with an overactive Ras signaling pathway, a common feature of many cancers. Because healthy cells still have their defenses intact, the virus leaves them alone and concentrates its attack on tumor tissue. HSV-1 is particularly useful for this work because its large genome contains many non-essential genes that can be swapped out or removed, giving scientists room to customize it.
Delivering Gene Therapies
Viruses are nature’s most efficient delivery vehicles for genetic material. That’s exactly what makes them dangerous as pathogens, but it also makes them invaluable for gene therapy. Scientists strip out the harmful genes from a virus, replace them with a therapeutic gene, and use the virus’s natural ability to enter human cells and deposit its genetic cargo.
The three main viral platforms used for this are adenoviruses, adeno-associated viruses (AAVs), and lentiviruses. Each has different strengths. AAVs are small and trigger relatively mild immune responses, making them well suited for long-term gene correction. Three AAV-based gene therapy drugs have reached the commercial market worldwide, treating conditions ranging from inherited blindness to spinal muscular atrophy. These engineered viruses treat monogenic diseases (conditions caused by a single faulty gene) with life-altering outcomes for patients who previously had no effective treatment.
Fighting Antibiotic-Resistant Bacteria
Not all lab-made viruses target human cells. Bacteriophages are viruses that infect and kill bacteria, and scientists are now engineering them to combat drug-resistant infections that antibiotics can no longer touch. This is one of the more creative applications of virus-making, essentially turning one microbe against another.
The crPhage trial, launched as the world’s first clinical trial for a recombinant phage therapy, combined a bacteriophage with CRISPR gene-editing technology to create a drug called LBP-EC01 that targets E. coli through two simultaneous genetic attacks. Another trial, SNIPR001, used engineered phages to deliver CRISPR components that destroy essential genes in E. coli strains causing urinary tract infections. In one notable case, a cocktail of phages was used to treat a patient with a widespread drug-resistant Mycobacterium abscessus infection, a situation where conventional antibiotics had failed.
Predicting Future Outbreaks
This is where virus creation gets controversial. In gain-of-function research, scientists deliberately enhance certain properties of a virus, sometimes making it more transmissible or better at evading immunity, to understand what nature might produce on its own. The goal is to stay ahead of evolution rather than react to it after a pandemic has already started.
During an outbreak of H5 bird flu viruses in Cambodia, gain-of-function research identified genetic markers suggesting the circulating strain could potentially transmit more easily between mammals (at least in ferret models, the standard lab stand-in for human respiratory infections). That finding was the persuasive factor that pushed authorities to begin developing a vaccine before the virus became a broader threat. Gain-of-function studies have also been used to identify bat coronaviruses capable of infecting human cells. While computer models and structural analysis confirmed one bat virus as a potential risk, only gain-of-function experiments using chimeric (hybrid) viruses identified an additional bat coronavirus that could pose a threat to humans.
Serial passage experiments, where a virus is grown repeatedly in cells or animals to see how it evolves, help scientists understand how viruses jump between species and which mutations drive that process. Studies passaging West Nile virus and rabies virus through animals have improved understanding of natural selection in RNA virus populations and cross-species transmission. Researchers also use these methods to map how viruses evolve inside a single host over time, information that’s valuable for tracing transmission chains during the early stages of an epidemic and developing better antiviral treatments.
Proving What’s Possible
In 2002, researchers synthesized a functional poliovirus entirely from scratch, outside of any living cell, using only its published genetic sequence and commercially available chemicals. The experiment was not designed to create a weapon. Its purpose was to demonstrate that viral genomes could be built from nothing, proving that eradication strategies for diseases like polio must account for the possibility of a virus being recreated from its known blueprint. Synthesizing viral genomes also provides a powerful tool for studying how individual genes contribute to a virus’s behavior and its ability to cause disease.
How This Research Is Regulated
The Biological Weapons Convention, an international treaty, prohibits the development, production, acquisition, transfer, stockpiling, and use of biological weapons, including engineered pathogens. On the research side, the U.S. Department of Health and Human Services maintains a framework specifically for studies expected to create enhanced potential pandemic pathogens (ePPPs). This framework requires a multidisciplinary review before funding is approved, weighing the potential scientific and public health benefits against biosafety and biosecurity risks. As of May 2025, a new executive order instructed federal agencies to revise this oversight policy.
The most dangerous work takes place in Biosafety Level 4 (BSL-4) laboratories, which maintain negative air pressure relative to surrounding areas so that air always flows inward, never outward. These facilities must document all design parameters and operational procedures, undergo verification testing before they open, and pass annual re-inspections to confirm containment standards are being maintained. The physical structure, ventilation systems, and operational safeguards must all be sufficient to contain whatever pathogen is being studied.
The tension at the heart of this field is real. The same knowledge that helps scientists build better vaccines and predict pandemics could, in theory, be misused. Proponents of gain-of-function research argue that the inability to predict how a virus will behave is precisely why these studies must continue, so that gap in understanding can eventually be closed. Critics counter that the risks of an accidental lab release or deliberate misuse outweigh the benefits. That debate remains unresolved, but the work itself continues under increasingly layered oversight.