The main thing that stands between the human species and the creation of a supervirus is a sense of responsibility among individual biologists.
– Richard Preston, The Demon in the Freezer, page 227

From a 2002 article by Danny Penman in The Guardian:

“A few months’ work in a makeshift laboratory is all it would take to produce a biological weapon that could kill hundreds of millions of people.

The scientific information is freely available and the raw materials easily sourced. The only difficult part would be mastering the necessary scientific skills, and they are taught on most biology degree courses.

One of the simplest ways of constructing a biological weapon would be to engineer an existing human disease and to make it even more lethal. Something as simple as the flu virus, when engineered with the gene for botulinum toxin, could wipe out a significant part of the human race. A low dose of this toxin is the main ingredient in cosmetic botox injections.

The genetic sequence for the toxin is freely available. This sequence could then be uploaded to a commonly available gene synthesizer, which would churn out millions of copies of the gene in a few hours. The flu virus would then be grown in the presence of this newly synthesized gene. As the virus reproduced, a few of the virus particles would absorb the gene. With a bit of luck, the budding terrorist would have produced a new biological weapon.”

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Human beings are inherently vulnerable to a significant number of lethal compounds. Given a highly contagious biological vector to distribute these compounds, the potential outcome is grim. Given adequate tools, knowledge, and time, groups could manufacture several such viruses and release them simultaneously in different areas, thwarting quarantine and antidote efforts. Governments know this — hence the Biological Weapons Convention. Bio-weapons are off-limits for warfare at the international level, but such rules might break down in the case of a large enough war, and will not be respected by rogue parties.