Longer than you feel sick - per the CDC, norovirus spreads during symptoms and for a few days after you feel fine.
During the sick stretch you are massively contagious, and it takes very few viral particles to infect the next person. The rules: no food prep for others during illness and for at least two days after, soap-and-water handwashing (sanitizer is a weak backup), and bleach-based cleaning of contaminated surfaces and laundry.
What to do
Common questions
How long is the stomach flu contagious?
Longer than the misery lasts - that is the answer most people get wrong. Per the CDC, most people with norovirus (the usual "stomach flu," no relation to influenza) get better within 1 to 3 days, but they can still spread the virus for a few days after they feel fine. During the sick stretch you are massively contagious - the vomit and stool carry enormous amounts of virus, and it takes very few particles to infect the next person. The practical rule set covers the whole household: the sick person stays out of the kitchen and does not prepare food for anyone during illness and for at least two days after symptoms end; everyone washes hands with soap and water after the bathroom and before eating (alcohol sanitizer is a weak backup against norovirus); contaminated surfaces and laundry get cleaned promptly with bleach-based products. Cruise ships and daycares are famous norovirus amplifiers for exactly these reasons. Pymander, a free AI doctor available 24/7 by text, can help you run the household containment plan.