Ceaser started dating Dr. Lee when she was still dating Tony, another of her co-workers. However, after she broke up with Tony and started dating Ceaser, she also started seeing another co-worker, Glenn. Ceaser found out about this and started poisoning Dr. Lee with cantharidin, which was once used as a crude aphrodesiac.
After she started showing symptoms, Ceaser tried to deflect suspicion from himself by saying Tony had taken the break-up badly. As such, the doctors did an environmental scan of Dr. Lee's house and Tony's house, but ignored Ceaser altogether.
However, the symptoms pointed to radiation sickness and Ceaser rightly pointed out that although Dr. Lee made bombs for a living, she would never work with radioactive material. Nevertheless, Dr. Lee was moved to an isolation room for fear her immune system was compromised.
When she started to get better, the team started to get suspicious, as whenever Ceaser had been with the patient, she kept getting sicker. When they finally hit on cantharidin as the cause of her engorged genital area, the only question was the source and Ceaser was the most likely poisoner.
As such, they set up a deception where they claimed that Dr. Lee had been diagnosed with radiation poisoning and told Ceaser he needed treatment too. When he resisted, the team realized the only way he could know for sure is that he knew what was causing her illness. They had already sent a team member to Ceaser's house to look for the cantharidin, but they asked him to confess to prove their diagnosis. He did so to avoid prison time for murder.