Chapter 19 of Contagion is where Jack begins to see his life with a new perspective. It starts when Jack is with Chet at Willow and Heath advertising agency and it ends in his apartment kitchen.
Colleen and Terese have asked Chet and Jack to stop by the ad agency after they finish work to get their reactions to the new campaign ads for National Health, the hospital that is the chief competition of Manhattan General. While there Jack begins talking about the latest outbreak at General (isn't a medcial examiner supposed to keep from disclosing these things?) of the third infectious disease: first, plague; second, tularemia; third, Rockey Mountain spotted fever. Being (slightly irrationally) pressed by Terese (who seems unable to follow logic all that well), Jack reveals his only half-conscious suspicion that the infectious diseases are being deliberately planted.
Part of his reason for suspecting this is that (1) the outbreak of three rare infectious diseases in (2) thee days (3) in a hospital environment (4) in the wrong season of the year defies probability. Another part of his reason is the existence in some victims of a high maintenance pre-existing medical condition: sever back trouble requiring multiple surgeries in Susanne Hard, asthma in Donald Lagenthorpe, diabetes in Mr. Nodelman. On his ride homeward through Central Park, the implications of having for the first time voiced his previously only half-conscious suspicions came home to him filling him with a sense of dread and ominousness. Simultaneously, he began to feel the dread and ominous threat of Central Park at night. Upon reaching home, he found that his apartment (now also looking unappealing and ominous) had been broken in to.
As Jack rode deeper into the dark, deserted [Central] park, its gloomy and somber views add to his disquietude. ... Jack streaked into a puddle of light beneath a lonely streetlight, braked, and skidded to a stop. He forced himself to turn around and face his pursuer ... he [then] understood that what was threatening him was coming from inside his own head.
While there was nothing to steal, all his belongs--such as they were--had been torn apart and tossed around. When he went to check the kitchen, he found the men who had broken in sitting around with their weapons, including semi-automatic weapons, gathered on the kitchen table. After inviting him to sit with them, the told him they'd been sent as a warning by someone who was unhappy about his visits to and questions as Manhattan General. To emphasize the warning the leader threw a "sucker punch" knocking to the floor and nearly unconscious. To supplement their payment for the hired attack, they took Jack's bike.