There are two reasons to stockpile food in preparation for a pandemic:
1. Having food at home will mean you don't need to go to a grocery store and risk getting infected and bringing the virus home.
2. Absenteeism and death among people who produce, process and transport food may lead to shortages.
Since local outbreaks of flu last between 6-12 weeks, many concerned citizens are storing food for 12 weeks, to avoid infection. The Department of Homeland Security has indicated that it may direct US citizens to remain in their homes for 90 days (12 weeks) during a pandemic. Concern about disruptions of the nation's food supply has led to others storing food for much longer.
From a series of flyers prepared by Melvin Johnson, Director, Lewiston-Nez Perce County Emergency Management: