Mark Alden Crislip (born April 25, 1957) is an infectious disease doctor in Portland, Oregon and former chief of infectious diseases at Legacy Health hospital system. Crislip has generated three podcasts, QuackCast, PusCast, and Gobbet o' Pus. A writer for medicine-related blogs, he has compiled his blog posts into several books. He co-founded the Society for Science-Based Medicine and served as president from 2013 to 2019.

Crislip was born in Cleveland, Ohio. He attended the University of Oregon from 1979 to 1983, where he earned a Bachelor of Science degree in physics. He then earned a Doctor of Medicine (MD) degree at the Oregon Health & Science University School of Medicine in 1983. He completed an internship and residency at Hennepin County Medical Center in Minneapolis in 1986, followed by a fellowship at Harbor–UCLA Medical Center. He is currently a board-certified infectious disease specialist at several medical centers in the Portland area.

Crislip was the producer and host of the podcasts QuackCast (2006–2019) and PusCast (2005–2021). His podcast Gobbet o' Pus was active as of 2022.

For each episode of QuackCast, Crislip delivered a monologue about a topic related to medicine, usually a critique of an alternative medicine practice or set of beliefs. He was inspired to create his own science-based medicine show after listening to the Slacker Astronomy podcast. The first episode of QuackCast was released on 5 May 2006. The podcast won three Podcast Awards in the Health/Fitness category, for the years 2009, 2010, and 2011.

Since 2009, Crislip has been producing the Gobbet o' Pus podcast which features short discussions of interesting cases he has encountered in his medical practice and other topics of interest to infectious disease specialists.

PusCast (also known as Persiflagers Infectious Disease PusCast) was a bimonthly review of the infectious disease literature that ceased in early 2021.

Crislip is listed as editor (emeritus) for the Science-Based Medicine blog where he regularly wrote posts on investigating the claims of alternative medicine until 2017. He is the co-editor, along with Steven Novella and David Gorski, of a 12-volume series of Science-Based Medicine Guides, based on posts from the Science-Based Medicine blog.

He writes posts for a Medscape blog called Rubor, Dolor, Calor, Tumor. Crislip compiled selections from his blogs into two e-books titled Puswhisperer: A Year in the Life of an Infectious Disease Doc (2014), and Puswhisperer Part Deux: Another Year of Pus (2016),