In Ghost Map Johnson devotes the final chapter to examining the threat of biological and nuclear terrorism that urban, metropolitan centers face with a focus on the role that forensics, epidemiology, public health, and computers might play in countering the threat. Potential threats to cities will take the form of a threat that specifically exploits the density of cities in order to harm us and create the most physical damage, with the least amount of effort, and provoke the most amount of fear. The revolution in information technology is partly the cause for the increase in potential threats that cities face. The information revolution is characterized by the shift from physical, weighted information, such as newspapers and books, to weightless information, such as online journals and newspapers. But it isn’t the information revolution alone that accounts for the increase in the potential threat to cities, but rather the shift from majority of the world population living in rural areas, to living in densely populated cities and urban areas. The increase in the density of metropolitan centers makes asymmetric warfare more deadly than before. Even though we have more advanced technology at our disposal to combat disease, disease today is more lethal because our planet is considerably more interconnected and densely populated than it was during the Broad Street cholera epidemic. Snow confronted the “fundamental perceptual limit of space” whereas we face the “perceptual limit of time”, in that we are tracking diseases that we can’t see because they don’t exist yet. It is vital that millions of dollars are devoted to research in the field of forensics, epidemiology, public health, and computers in order to prepare for the emergence of a future outbreak, epidemic, asymmetric warfare, and biological terrorism because if it appears and starts spreading it will have devasting effects due to our densely populated urban centers and our interconnected world due to air travel. We have to have a continued commitment to public health institutions, commit to developing public health infrastructures in the developing world, and pattern recognition, local knowledge, and disease mapping remains essential. Countering the threat of nuclear terrorism has proven more difficult because there is no way to disengage nuclear and explosive weapons once they have been detonated and because of the breakup of the breakup of the Soviet Union, an increase in technological expertise and Iran’s renewed commitment to a nuclear program. The key to countering the threat of nuclear terrorism is to continue to advance our technology for detection, that nation-states continue to ban the building of new nuclear weapons, and they work on eliminating the ones already in place. The threat of biological terrorism can be countered by a continued commitment among nation-states to ban the state use and development of biological weapons. These are possible roles that forensics, epidemiology, public health, and computers can play in countering the threat of biological and nuclear terrorism but there are simpler solutions that are often overlooked but vital in countering these threats; the acknowledgment of the underlying problem, listening to science and not superstition, the rational application of the scientific method to public-health matters, and we must keep a channel open for dissenting voices to prevent being under the spell of a theory and over determination. Early detection and planning for the emergence of a potential threat that would take advantage of the density of cities is key to countering the threat because “the greatest risk of deliberately planned urban epidemics is that we won’t recognize the outbreak until it is too late for a vaccine to stop the spread of the disease”. Time is of the essence in countering the threats to densely populated areas because the epidemic can spread quickly due to the interconnectivity of our world due to air travel and the density of our urban centers. In order to combat modern threats to cities we would have to employ the 21st century version of Snow’s map: “making visible patterns in the daily flow of lives and deaths that constitute metabolism of a city, rising and falling fortunes of the sick and the healthy.”