Biological weapons can kill more people than chemical weapons. But the United States is unprepared to deal with a biological attack. Cities have made the largest strides to be prepared, but much of the needed equipment, such as pathogen detectors, is not in place. Medical supplies may not effectively treat symptoms. Emergency medical response, such as specialized hospital therapy, is limited. Biological weapons can range from salmonella, which can incapacitate a portion of the population, to bubonic plague, which can be engineered for mass casualties. Technical expertise about biological processes have become much more widespread. Creating biological agents is demanding but not a great technical challenge. Terrorists may want to use biological weapons because they can be in a small package, yet have extreme destructive potential. Their incubation period can allow perpetrators to leave the affected area long before anyone knows an attack has been carried out. Terrorists' goals have shifted lately from making a political statement to maximizing damage to the target as an end to itself. Motivations also include religious or ethnic considerations or instilling fear. To demonstrate credibility, a biological attack can have a disproportionate impact. Mother Nature can be a terrorist as well. Since infectious diseases were widely dismissed as a world health threat 30 years ago, nature has produced 30 new diseases. Therefore, health security and national security overlap. If the United States can prepare for intentional releases of pathogens, it can be ready for threats from nature. Effective multipurpose drug treatments would help in the battle against naturally occurring and intentional releases of infectious diseases. Limiting vulnerability is the most promising way to prevent or mitigate a biological attack on the United States. Figure, table, references