The world has been free from smallpox since 1980. The last outbreak occurred in 1977, it was successfully contained, and the eradication campaign passed into the history books as one of the 20th century's greatest achievements. Governments then wound down their vaccination programmes, and the human race moved on, untroubled by Variola major but also increasingly defenceless against it as the immune populations died out. Last Tuesday, a BBC broadcast dramatised what would happen if a terrorist deliberately unleashed a virulent strain of smallpox on a now virgin population.

Smallpox 2002—Silent Weapon is a fictional account of a smallpox pandemic in which 60 million people die and three times as many become ill. Civil unrest, martial law, and economic collapse follow the virus across the globe as civilisation's infrastructure collapses under the weight of the dead and dying. The story begins in New York with a single case. An uncontrolled epidemic quickly follows because doctors don't recognise the disease, and officials don't act fast enough when they do. Within a few days smallpox spreads across the Atlantic, then to the rest of the world. The devastation is greatest in Africa, where the combination of AIDS and smallpox kills nine tenths of those infected.

Slowly, it becomes apparent that the outbreak was an act of terrorism. Investigators find an infected body in Grand Central Station, then a key to a hotel room, then a Bible open at Ezekial, chapter 5, verse 12: “A third of thee shall die with the pestilence.” The lethal pandemic was caused by a single Godfearing American with a grudge. DNA fingerprinting of the virus type found on his body leads the investigation to a Russian laboratory where military leaders stockpiled the virulent strain “India 1” during the cold war. It's not clear how the terrorist acquired his supply, but the disturbing truth is that Russia's economic collapse at the end of the cold war made its military laboratories decidedly leaky.

The BBC billed the programme as a docu-drama, a sometimes confusing hybrid of two very different styles. The documentary label encourages viewers to believe what they see, while the term “drama” gives the producer licence to embellish the facts for dramatic effect. It's a powerful if slightly dishonest combination, and one that kept me glued to the screen until the end of the 90 minute broadcast. Intrusive questions then disturbed my daily routine for several hours. Did the Russians really stockpile smallpox virus India 1? Is there really an army of disaffected Russian scientists selling smallpox to the highest bidder? And can you really culture smallpox in the kitchen with a home brewery kit?

Telltale shots of New York's World Trade Center indicate that the programme was conceived and produced before the terrorist atrocities of 11 September, but the subsequent paranoia about terrorism must have increased its impact. Viewers who find themselves unable to sleep might be reassured, as I was, by the US Centers for Disease Control and Prevention. On the website are detailed plans for the management of a terrorist attack with smallpox, mathematical models of an epidemic's likely behaviour, and the promise of enough smallpox vaccine for everyone in the United States by the end of 2004. All documents have been updated since 11 September. If there is a madman out there with a Bible and a Petri dish the Americans, at least, are ready for him.