The Bush administration has warned that recent intelligence suggests a high risk of terrorist attacks against U.S. targets. Following are some of the weapons that experts fear may be used by terrorists:

Biological Weapons

Organisms or toxins that can cause deadly disease in people, livestock and crops. There are different types, including bacteria, viruses and toxins, often colorless and odorless. The size of these agents -- five microns, or less than one-fifth the width of human hair -- means they can remain airborne for hours to a day or more. They can bypass the filtering mechanism in a human's upper respiratory system and enter the lungs and the bloodstream. Among those U.S. officials most fear are anthrax and smallpox.

Anthrax: Caused by a bacteria, it can cause illness in three ways: cutaneous, gastrointestinal and inhalation, the most deadly form. In 2001, anthrax was turned into a powder and sent in letters through the U.S. mail system. Twenty-three people contracted it, and five died. The perpetrator has not been caught.

Smallpox: Caused by a virus known as variola, it is spread most often by an infected person releasing saliva droplets from the mouth into the air, which is then inhaled by another person. The disease was eradicated in late 1970s, but officials fear terrorists may use it as a weapon. There is no specific treatment, and the only prevention is vaccination. President Bush recently ordered that some 500,000 U.S. troops deployed in high-risk parts of the world be vaccinated against smallpox. He was also vaccinated. Public health care workers, emergency room doctors and other officials also are to be vaccinated.

Other biological weapons include plague, an infectious disease of animals and humans caused by the bacterium Yersinia pestis, which comes in three forms; botulism, a muscle-paralyzing disease caused by a nerve toxin produced by the bacterium Clostridium botulinum; and tularemia, caused by the bacterium Francisella tularensis. These can be treated but are lethal without immediate medical attention.

Chemical Weapons

These are poisonous gases, liquids or solids that have toxic effects on people, animals and plants. They are characterized by the rapid onset of medical symptoms and easily observed signatures, such as colored residue, dead foliage, pungent odor and dead animals and insects. Most chemical weapons cause serious injuries or death.

Sarin: A manufactured compound that is toxic to humans, it is colorless, odorless and tasteless and occurs both as a liquid and vapor. Inhaled or absorbed through the skin, it can suffocate its victims by paralyzing the muscles around their lungs.

VX: A nerve agent that disrupts the transmission of nerve impulses in the body. The most potent of all nerve agents and the least volatile, it is about 50 times more toxic than cyanide gas. A dose that would be lethal to 50 percent of the people exposed is about 10 milligrams, a tiny amount that could be held on the end of a straight pin.

Mustard gas: A liquid that can be vaporized to form a gas. It attacks the skin and eyes and, if inhaled, can damage the lungs and other organs. It enters the body through inhalation or skin contact and damages any tissue it touches.

Radiological Weapons

A radiological dispersal device, known as a "dirty bomb," is a conventional explosive such as dynamite packaged with radioactive material that scatters when the bomb goes off. The device is designed to injure or kill by creating a zone of intense radiation that could extend several city blocks. People in the immediate vicinity of the blast would be killed, and radioactive material would be dispersed into the air and reduced to relatively low concentrations. Many experts say very few people would die or become sick from the radiation exposure.

-- Valerie Strauss