While working, Kasie’s lab gets put on lockdown, which she calls her “third strike” after a challenging year. To make matters worse, she and Jimmy soon realize the biohazard that caused the lockdown is deadly and can result in lethal symptoms.
As the episode progresses, Kasie begins showing symptoms of having inhaled the toxins. Soon, she starts coughing up blood and eventually passes out after leaving her mother a goodbye voicemail.
Source: https://www.distractify.com/p/is-diona-reasonover-leaving-ncis