The loss of more than 3,000 people to al-Qaeda terrorism on September 11, 2001, brought to many Americans the sudden recognition that their country was no longer leading a charmed life. But a number of the nation's security experts had seen it coming. In 1999, for example, a commission led by Senators Gary Hart and Warren B. Rudman examined U.S. security policies and, in a report published two years before the al-Qaeda attack, concluded, "There will...be a greater probability of [catastrophic terrorism] in the next millennium....Future terrorists will probably be even less hierarchically organized, and yet better networked, than they are today. Their diffuse nature will make them more anonymous, yet their ability to coordinate mass effects on a global basis will increase....The United States should assume that it will be a target of terrorist attacks against its homeland using weapons of mass destruction. The United States will be vulnerable to such strikes."

The concept of megaterrorism was well known. The warning was there. Only the date, place and nature of the deed were in question to those who had looked at the prospects. And though last September's slaughter was not caused by the "weapons of mass destruction" of which the Hart-Rudman Commission warned, it is still my belief that the biggest threats we face are two types of such weapons: biological and nuclear devices.

I am, unfortunately, no stranger to either. My work on nuclear weaponry began at the Los Alamos National Laboratory in 1950 and continues to this day. Through many years of service on the White House President's Science Advisory Committee and its many panels, and with similar bodies of the U.S. Defense Department and the U.S. Department of State, I have become familiar with the status of nuclear weapons around the world. And in 1998 I served with Donald Rumsfeld, now the secretary of defense, and seven others on the Commission to Assess the Ballistic Missile Threat to the United States, which by law had access to all intelligence regarding not only foreign missiles and nuclear weaponry but bioweapons as well.

Biological-warfare agents are, in my judgment, the biggest menace we currently face, but not all such agents are created equal. Bioweapons perfected by the major powers in the immediate postwar period included diseases of plants, animals and humans. They were further divided into diseases such as anthrax that are merely infectious-caused only through direct exposure to a weaponized bacterium or virus-and those like smallpox that are also contagious, or spread from one person to another.

The five deaths and six inhalational anthrax illnesses caused by anthrax-bearing letters-probably from a single domestic terrorist incited by the events of September 11-have taught us a lot about inhalation anthrax that we did not know before. Quite apparent, and not surprising, is the fact that the spores may reside for weeks and months in the lung before vegetating-even in the presence of effective antibiotics-and if antibiotics are withdrawn, an ensuing infection is usually lethal within days unless promptly treated. The likelihood of prompt treatment has, of course, increased greatly, since a disease almost unknown to the ordinary medical practitioner is now at the top of everybody's alert list. Yet anthrax remains a concern because in the form of spores it is so durable. The dissemination of dry spores has been achieved in Russia and the United States and perhaps elsewhere, and tons of anthrax spores have been produced and weaponized-for instance, in 1991 in Iraq.

But my chief concern is not a handful of anthrax letters, or even a hypothetical mass mailing of 10,000 anthrax letters. What worries me most are biological agents that are contagious as well as infectious.