Cell by Robin Cook
George Wilson is a medical resident in a busy hospital radiology department.  While he is starting the fourth year of his residency, two of his patients die unexpectedly soon after being diagnosed with a terminal illness.  George discovers that they both, as well as George's deceased fiance, had been patients of iDoc, a cutting edge medical app developed by Amalgamated Healthcare and a old college girlfriend of George's.
George puts his residency and future in jeopardy when he begins to investigate these deaths which he alone believes there is a connection between.
I have enjoyed Robin Cook's medical thrillers before, and Cell lives up to his other books.  The story grabs your interest immediately, keeps up the quick pace, and doesn't let you put it down until the surprising conclusion.