Democratic presidential nominee Hillary Clinton was supposed to be in California this week. Her itinerary included a fundraising concert Monday with k.d. lang in San Francisco; she was set to appear in LA at a lunch with singer Lionel Richie at the home of Seth McFarlane, and a dinner at the Beverly Hills home of Diane von Furstenberg and Barry Diller.

But the candidate's trip to the Golden State is now off after she fell ill with pneumonia. 

For a look back at how health issues have affected presidential campaigns, Take Two spoke to Julian Zelizer, professor of history at Princeton University.

Press the blue play button above to hear the interview.