To a post-9/11 lexicon of phrases like "threat level" and "homeland security," we need to add another: food defense. The possibility that the nation's food supply could be targeted by terrorists has existed since at least the anthrax letters of October 2001, but recent events have underlined just how real the threat is. Suspects in last month's failed car bombings in London and Glasgow, for instance, include physicians, a reminder that terrorists can have biomedical know-how. And imports of contaminated food from China—pet food laced with the chemical melamine, toothpaste with the poisonous compound diethyl glycol and seafood with carcinogenic antimicrobials—"show how vulnerable the food supply is to intentional acts of terrorism, too," says Frank Busta, codirector of the National Center for Food Protection and Defense at the University of Minnesota.

In 2001, the then Secretary of Health and Human Services Tommy Thompson told Congress that food inspections were so porous that the possibility of intentional contamination made him "more fearful ... than anything else." In response, Congress gave the Food and Drug Administration (FDA) more money for inspectors and passed the Bioterrorism Act of 2002 to stop or catch attacks on the food supply. But the 600 additional inspectors for imported food are gone, putting us back down to 2001 numbers, and the 2002 law failed its first real test when it took weeks to identify melamine as the pet-food culprit. "The progress we made after 2001 was short-lived," says Caroline Smith DeWaal of the Center for Science in the Public Interest. "The intent of the person adding melamine to the wheat gluten was not as nefarious as that of a terrorist, but the effect could have been just as bad if a more toxic agent had been used [in human food]."

The industry has not been standing idly by. Food producers have implemented systems to identify and secure points of vulnerability, such as storage tanks or transport. Although the steps are voluntary, and there are no reliable records on which companies are doing it right, Allen Matthys of the Grocery Manufacturers Association says that "individual companies have done a lot of work behind the scenes to be sure their products are safe and secure." Clearly, though, not all the holes have been plugged. In 2003, a Michigan supermarket worker contaminated 200 pounds of hamburger with a nicotine-based pesticide, making 92 people sick. The year before, school lunches were accidentally contaminated with ammonia at a warehouse, sending 44 people to hospitals. It is "simple," says the FDA, "for one person to intentionally contaminate the food supply and have a major impact."

Food-defense experts say the list of foods in which an attack could have what FDA calls "catastrophic consequences" begins with those made in large batches; poisoning a 5,000-gallon kettle of pasta sauce is easier and would cause more widespread harm than poisoning 40,000 individual bottles of artisanal honey. Other prime targets: foods subject to uniform mixing, such as milk and liquid eggs, since contaminating one of the multiple sources that go into the batch would poison the whole thing and be tough to trace. Foods with a short shelf life, such as ground meat, would likely be in people's stomachs before health officials identified the cause of a mass poisoning, making such perishables attractive targets.

Matthys notes that contaminants often change the taste, odor, consistency, acidity and other qualities of food, and so could be detected at the plant. Strong flavors or odors, however, such as in fish sauce, as well as certain textures, such as ground meat and intense colors as in soy sauce, are more likely to "conceal the presence of a contaminant," says the FDA. Not every batch of food is tested, of course, and tests are for specific agents, not some generic "poison." As shown by the time it took to ID the melamine in pet food, says Shaun Kennedy of the Minnesota center, "we look for horses, not zebras," so a novel poison could be widely distributed before the food carrying it was identified.

The list of chemicals that could poison food is almost endless. Among microbes, the most easily obtainable and pathogenic include salmonella, E. coli O157 and Clostridium botulinum. Canning kills all known biological agents, including anthrax spores and botulinum, but many chemical agents survive heat, Busta notes. Particularly attractive agents are those that take many days to cause illness, such as the mushroom toxin alpha-amanitin: the lag between eating and illness "could minimize opportunity for public health intervention," as the FDA delicately puts it.