A cluster is defined as five or more cases in close proximity.

http://acg.instructure.com/eportfolios/54522/TrenAQ_50_Mg_Dose__Magnum_Pharmaceuticals__Injectable_Steroids
http://learn.canvas.net/eportfolios/12631/Asep_Mubarok_2020__German_Remedies__Allergies

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University leaders say they have moved 80 percent of classes online, are encouraging faculty and staff members to work remotely if possible, and have de-densified dorms.