In October 1979, a top-secret CIA intelligence report featured the first inklings in the West that something unusual and disturbing had allegedly taken place in the Soviet Union several months earlier.

In April of that year, patients started appearing at hospitals in the industrial city of Sverdlovsk, now known as Yekaterinburg, in the Ural region of the Soviet Union. They were showing symptoms of what doctors first thought to be an unusually virulent and deadly form of pneumonia. It wasn’t.

The outbreak that ultimately killed more than 60 people was in fact caused by anthrax spores that had been accidentally released from a Soviet biological weapons facility. How exactly this happened is still unknown.

Officially, neither the facility nor the Soviet bioweapons program was supposed to exist; a few years earlier, Moscow, along with Washington, had ratified a landmark international treaty prohibiting biowarfare work. When US officials publicly raised questions about the incident at Sverdlovsk, the Soviet government denied any biological weapons research was taking place, blaming the outbreak on contaminated meat. It wasn’t until 1992, after the fall of the Soviet Union, that Russian President Boris Yeltsin acknowledged that the incident was the result of a covert bioweapons program.

How is it possible that a bioweapons accident that killed dozens was kept secret for decades, even in the Soviet Union? As the Washington Post reporter David E. Hoffman writes in The Dead Hand, his history of the Cold War arms race, the answer lay in the nature of the weapons themselves: “Biological weapons were the ultimate challenge for spies, soldiers and scientists.”

Unlike a missile silo, easily distinguishable from the air, a laboratory where bioweapons are being developed doesn’t look that different from a benign medical laboratory. Unlike nuclear warheads, which leave clear radiological traces in their silos and are unmistakable in their use, a weaponized pathogen and the outbreak it would cause could be difficult to discern from a naturally occurring one, giving any attacker plausible deniability.

The mystery surrounding these weapons is just as much a problem today as it was during the Cold War. Putting aside the still politically fraught question of whether Covid-19 escaped from a Chinese lab or, like most outbreaks, jumped from animals to humans naturally, the bigger problem is the simple fact that we may never know for certain.

“What the pandemic tells us is that nobody can do attribution,” said Drew Endy, professor of biological engineering at Stanford. Intelligence agencies have determined that Covid was not a deliberately engineered bioweapon, but the confusion about its origins does suggest that if an even more virulent, intentionally designed pathogen were to be unleashed, it might be very difficult to say for certain who was behind the attack, or even whether it was an attack at all.

This kind of plausible deniability could make using such a weapon more attractive to attackers. Biowarfare is only set to become a bigger threat in the coming years if, as many experts predict, artificial intelligence makes it easier, cheaper, and faster to develop new biological compounds, including weaponized pathogens far more sophisticated and deadly than the anthrax that killed dozens in Sverdlovsk 46 years ago. That’s why Endy, a pioneer in the field of synthetic biology — the construction of new biological systems or deliberate alteration of existing ones through genetic manipulation — argues that new forms of detection are desperately needed for this new threat landscape.

“When the Iron Curtain came down, we found it useful to have geospatial intelligence to see what was happening on the other side regarding nuclear weapons,” he told Vox. “Today, there’s a molecular curtain. The stuff that’s invisible, that we can’t see, is all around us and could be harmful. And we don’t really do that kind of intelligence.”

The technologies that could allow adversaries to create ever more dangerous bioweapons are advancing at a much faster clip than defensive measures. But at the moment when AI might be amplifying the risks of this type of weapon, it may also be emerging as the key for detecting and stopping them.

Germ war is nothing new, but the threat is changing

Biological warfare dates back at least as far as the 14th century BC, far before anyone knew that germs caused disease, when the Hittites sent diseased rams to their enemies to infect them with the dangerous bacterial infection tularemia. Every major combatant in World War II had a biological weapons research program — including the US — and Japan even deliberately unleashed germs in China.

A history of germ violence

Warfare and disease have always gone together; until the 20th century, illness was responsible for killing more soldiers than weapons in many conflicts. Even today, bullet and shrapnel wounds in the war in Ukraine have become breeding grounds for drug-resistant bacteria. The deliberate use of illness as a weapon also has a long history.

In the 14th century BC, the Hittites sent diseased rams to their enemies to infect them with tularemia, a dangerous bacterial infection still classified as a potential bioweapon by the Centers for Disease Control and Prevention today. British soldiers infamously gave blankets infected with smallpox to American Indian tribes in the 18th century. During World War II, Japan’s........

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