WHEN the terrible sight of two airliners crashing into the Twin Towers shook the world on September 11, 2001 we were forced to open our eyes to terrorism, writes Angela Kelly.

That was, we were convinced, as bad as it gets.

But, in the aftermath of that date, we have had to acknowledge the possibility of an even more frightening threat: bioterrorism.

At 10am on October 15 that same year, a series of anthrax attacks through the American postal system caused mayhem. This threw up an even worse spectre. Alongside this threat, another existed which made anthrax pale into insignificance. Smallpox. Suddenly, we all became aware of our vulnerability to biological weapons, and, as writer Richard Preston reveals, we are absolutely right to do so. For the real fear now is a genetically engineered strain of smallpox that would be immune to vaccines. The result could decimate the population and, like a fire that refused to respond to water, it would simply spread and spread.

To understand why governments rate this particular bio-weapon so highly, Preston takes us back to the time before 1979 when the World Health Organisation officially declared that smallpox had been eradicated from the planet. Before then, two million people a year were dying of smallpox. Only the work of dedicated scientists achieved its end, but did that mean, therefore,that the virus was also dead and gone?

The answer to that lies in two high-security freezers -- one in Atlanta and one in Siberia -- where supplies of the smallpox virus are still stored. Science, the need for answers, and the reality of a suitable vaccine still demands it. But, what is increasingly worrying world governments is that there are almost certainly illegal stocks of smallpox virus in hostile states, including Iraq and Korea.

Genetic engineering there could well have created a superpox, resistant to known vaccines. So, even as you read these words, and while weapons' inspectors are still hunting for any evidence of Saddam Hussein's possible weapons of terror, experts are desperately searching for ways to combat what is now seen as a very real threat to humanity.

Smallpox cases have been recorded for centuries. The virus interacts with the victim's immune systems in various ways, triggering different forms of disease in the human body.

Classic ordinary smallpox comes in two basic forms: the discreet type and the confluent type.

Discreet ordinary smallpox involves a general feeling of being unwell with a slight temperature, painful pustules appear on the skin as separate blisters. The patient has a chance of survival.

In confluent ordinary smallpox, blisters merge into sheets and it is typically fatal.

In haemorrhagic smallpox, the skin remains smooth, but it darkens until it can look charred.

In the old days, doctors used to call it black pox. The whites of the eyes can be ruby red and, if the patient lives long enough, the whites turn black. There is likely to be bleeding from the extremities, and the immune system goes into shock. It is virtually one hundred per cent fatal.

Preston's compelling book takes us not only through what has already happened -- the outbreaks and how they were dealt with, the individual scientists whose genius has already made a difference, and the regimes that have encouraged the creation of bio-weapons -- but he centres on what is happening right now in America. Here, leading virologist Peter Jahrling at the US Army Medical Research Institute of Infectious Diseases (USAMRIID) is urgently researching the development of a drug that will combat the virus. Controversially, they are working with live smallpox.

Preston explains exactly what may be at stake if Jahrling's bold experiment fails, and smallpox is unleashed on the planet once more.

This is no cosy read. But, in the light of the daily headlines in the news, and with the indelible memory trace of terrorist atrocities already perpetrated, it is a book that will leave you thinking deeply long after the final page.

Published by headline at £17.99.