Answer: December-April is the worst but it can occur all year round.
According to the CDC, rotavirus technically occurs from November-April1. However, my son got rotavirus in the middle of May. Norovirus gastroenteritis occurs all year round. I have no proof but I am fairly certain that the number of visitors to this website reflects the amount of stomach flu going around in the United States. The traffic is highest to this website December-April. Traffic is 3-4 times lower May-November. Even though, there seems to be the most stomach flu December-April, my family had a bad stomach bug in June so it is never actually gone. --Annie Pryor, Ph.D.
It is very important to clean with a product that actually kills norovirus when a stomach bug is in your home such as the Clorox® Hydrogen Peroxide Spray and Clorox Hydrogen Peroxide Wipes.