The FBI is called in to help trace discretely and hopefully contain a fatal infection. Thus Don learns to his surprise that Charlie actually already has even higher security clearing than his big brother, being an NSA consultant. In collaboration with the Atlanta disease control center, the epidemic is diagnosed to be one, or as Charlie later helps discover two, vicious strains of the Spanish flu which wiped out more people then the first world war. For security reasons, the brothers can't tell why they try to keep dad away from the city's most endangered quarters. Charlie's brilliant chaos theory application is their only change to work out patient(s) 0, the source, and spread pattern. Don concentrates on bio-tech firms which almost certainly provided the virus, and the harder, open question of motive, yet key to arresting the culprit.