Any kind of physical trauma—surgery, a car accident, or a severe illness, even the flu—can cause temporary hair loss. This can trigger a type of hair loss called telogen effluvium. Hair has a programmed life cycle: a growth phase, rest phase and shedding phase. “When you have a really stressful event, it can shock the hair cycle, (pushing) more hair into the shedding phase,” explains Marc Glashofer, MD, a dermatologist in New York City. Hair loss often becomes noticeable three-to-six months after the trauma.