Lehigh County Executive William Hansell has resigned his post, effective immediately.
Hansell was appointed to fill Don Cunningham's unexpired term last summer.
Hansell has recently been hospitalized with pneumonia and subsequently diagnosed with non-Hodgkins lymphoma, according to a statement from the county.
"I have been proud to serve as Lehigh County executive for the past year," Hansell wrote in his resignation letter. "I felt that it was the culmination of my 52-year career in local government. The decision to resign has been one of the hardest I’ve had to make in my professional life."
Thomas Muller has been appointed acting executive. Muller has spent the past seven years serving as the county's director of administration.
"I have every confidence in Tom’s ability to carry out the responsibilities of this position and to serve all the residents of Lehigh County," Hansell said.
Muller is also the Democrats’ unopposed nominee for the office. He will run this fall against either Scott Ott or Dean Browning, the two Republicans up against each other in next week’s primary.