They called it Clade X. A deadly spin on influenza that borrowed some nasty tricks from Nipah virus, it first popped up in Germany and Venezuela. Within a year, 150 million people were dead.

Luckily, Clade X isn't real. It's a made-up virus, deployed in a simulation with US government officials last month, to test how the world might respond to a pandemic engineered and unleashed by bio-terrorists. But while Clade X might be fictional, the threat is scarily genuine.

Now, a new scientific report commissioned by the US military has assessed and ranked the most serious vulnerabilities posed by this emerging field of synthetic biology – tools like CRISPR, which give scientists the power to edit and engineer whole life forms, but could prove deadly if misused.

"In and of itself, synthetic biology is not harmful," explains microbiologist Michael Imperiale from the University of Michigan, who chaired the National Academies of Sciences, Engineering, and Medicine committee behind the report.

"The level of concern depends on the specific applications or capabilities that it may enable."

Those capabilities could be devastating. Last year, scientists in Canada provoked outrage when they genetically stitched together the extinct horsepox virus in the lab.

This accomplishment, the team claimed, could aid vaccine and cancer research – but other researchers condemned their work, claiming it made the world more vulnerable to smallpox.

According to Imperiale, since the genetic code of practically any mammalian virus can now be found on the internet and synthesised, the feasibility of recreating known pathogenic viruses like this makes it of the highest concern in terms of synthetic biology threats.

"The technology to do this is available now," Imperiale told The Guardian.

"It requires some expertise, but it's something that's relatively easy to do, and that is why it tops the list."

Also of high concern is devising ways to make existing bacteria or viruses more dangerous – such as by intentionally encouraging antibiotic resistance, genetically altering their toxicity, or by strengthening their immunity to vaccines.

Another threat could be exploiting microbiota in the human gut, to somehow create dangerous biochemicals that could be passed between people.

That might sound like science fiction, but it could soon be within the realm of possibility, the researchers think, as technology and scientists' understanding of synthetic biology evolve.

"There are certain capabilities that may not be possible now," Imperiale told NPR, "but in those cases we tried to identify what the bottlenecks or barriers might be that, if overcome, would enable those to be more possible."

According to the report, mid-level threats could include attacks designed to modify the human microbiome, immune system, or human genome.

The team ranked the seriousness of vulnerabilities based on things like the technology used to implement such attacks, the expertise needed, and the relative strength of countermeasures available to authorities to mitigate harm.

Because of some these factors, the difficulties of developing a largely new pathogen – like the fictional Clade X – as opposed to merely augmenting an existing threat, mean we might have less to fear from 'mix and match' bio-threats.

"Even simple changes to existing viruses can produce drastic deficiencies in key viral properties," the report notes, "making any such effort especially difficult."

Also difficult might be swallowing some of the apparent hypocrisy that hangs over the report's origins.

While the analysis was commissioned by the US Department of Defence, others are quick to point out it's the American military that's actually a chief funder of synthetic biology research, while simultaneously hosing down the dangers as "not a major threat issue at the moment".

According to some commentators, that uneasy state of affairs may be just another potential threat we need to keep our eye on.

"You don't want to start a new bioweapons race," biosecurity researcher Filippa Lentzos from King's College London in the UK, who wasn't involved with the report, told MIT Technology Review.

"The field needs to ask itself who is driving the agenda, and how does this look from the outside."

The findings are published in the report, Biodefence in the Age of Synthetic Biology.