The judge posted on Facebook on Thursday, saying she was experiencing a sore throat and fatigue and was not in the best of health during a commissioner's meeting where she was seen wearing a K95 mask.
She said that she got herself checked out and said her test results came back later and revealed the mononucleosis diagnosis.
"I share this because I will be less visible around the community for the next several weeks while I recover, and I want to be fully transparent about why," she said.
According to the Centers for Disease Control and Prevention, infectious mononucleosis is a contagious disease caused by EBV, or the Epstein-Barr virus. It is spread through bodily fluids, especially saliva. It also takes at least four to six weeks to recover from it.
Hidalgo said that although she would be absent from her duties, her team would represent her when necessary.