This is a very complex question because there are many many characters who are roughly divided into three groups: (1) the group of medical and advertising professionals with whom Jack works or directly interacts; (2) the group of medical patients who are the unfortunate victims of the contagion spread by the infectious diseases; (3) two black ghetto gangs, Warren's and Twin's gangs.
While Jack was watching, Warren sank the final basket of the game and the losing team slink off the court, momentarily disgraced. As the new game was being organized Warren caught sight of Jack. He waved and strutted over, It was the winning team walk.
"Hey, Doc, whatcha know?" Warren asked, "You coming out to run or what?"
The group of patients consists of the victims of plague, tularemia and Rocky Mountain spotted fever. This group is further divided into two sub-groups: (a) the sub-group of people who check into the hospital for existing conditions or illnesses and subsequently contract the contagion; (b) the sub-group who worked in central hospital supply and contracted the same contagious diseases as the infected patients. For all but one of the workers in central supply, Jack has no direct contact with this group of characters.
The group of medical and advertising professionals with whom Jack works or directly interacts is structured in three sub-groups. These three are (a) the other medical examiners, laboratory technicians, support staff and administrators of the office of the medical examiner; (b) CDC staff, National Biologicals staff, the medical and laboratory staff at Manhattan General hospital; (c) members of the creative team at Willow and Heath advertising firm where Terese and Colleen work.
These groups interact in a miniature web-like manner. Jack interacts directly with other medical professionals, like Chet and Laurie, gang members, Gloria Hernandez from central supply who died after speaking with him, and advertising professionals. He interacts indirectly with the victims through autopsies and through his investigative conversations with their surviving family members. He interacts directly with the gang members when he plays basketball with them, when he has a hit-man after him, and at other times when he is being questioned by them.
One final and critical interaction is that between the villains, Terese and her brother Richard--who interact directly with each other--and between Richard and the victims. Richard interacts indirectly with each victim who dies because he puts the pathogens in the humidifiers that are sent (somehow, through somebody's orders, for some unstated medical reason) to each victim and with the women in central supply who clean the humidifiers. Terese interacts indirectly with the victims through her guilty knowledge of what Richard has done because she goaded him into helping her raise her client's market share through spreading artificially contracted nosocomial diseases.