William E. Gilli, M.D., F.A.C.P.


Dr. Gilli is a Bakersfield native. He attended West High School and graduated from the USC School of Medicine in 1983. Dr. Gilli is Board Certified in Internal Medicine. He is an Assistant Professor of Medicine at UCLA teaching at Kern Medical from 1993 until 2019. At Kern Medical, he has helped teach over 300 physicians for 26 years until retiring from Kern Medical in 2019. Since Retiring from Kern Medical, Dr. Gilli is working full time at Mesa West Medical. He is also a Fellow at the American College of Physicians. In early 2009, Dr. Gilli underwent a total hip replacement and has returned to running. He is an avid walker, runner, cyclist, and skier.

Tracy Simonsen ACNP-C

Nurse Practitioner

Tracy Simonsen received her Bachelor’s Degree from Cal State University Bakersfield in Nursing. She then worked in Intensive Care Medicine for the next 7 years at Kern Medical Center. She obtained her Master's Degree from UCLA in Nursing with a specialty as an Acute Care Nurse Practitioner. Tracy Simonsen has been at Mesa West Medical since 1996. Tracy enjoys tennis and golf.

Kevin Nelson, FNP

Nurse Practitioner (Retired)

Kevin Nelson obtained his Family Nurse Practitioner and Physician Assistant Certificates from Stanford University. Kevin Nelson worked at Mesa West Medical from 2002 until the end of 2019 when he retired. Kevin is currently enjoying his retirement in Grover Beach but will occasionally will be filling in for Dr. Gilli (when Dr. Gilli is off on vacation).