You are most contagious while symptomatic, especially while vomiting, and for the first few days after feeling better, but norovirus can keep spreading for 2 weeks or more after recovery.
The numbers explain the outbreaks: billions of particles shed per episode, and only a few needed to infect. The countermeasures are specific: soap-and-water handwashing, because alcohol sanitizer barely touches norovirus; bleach-based cleaning of any surface vomit or diarrhea touched; no cooking for others while sick and for at least 48 hours after symptoms end (longer per food-service rules); and hot-water laundry for soiled items. The medical danger is dehydration, not the virus itself: small frequent sips or oral rehydration solution, with care-seeking for very little urine, standing dizziness, or blood in vomit or stool. Expect 1 to 3 miserable days, then recovery. There is no antiviral for it; fluids and time are the treatment.
What to do
Common questions
How long am I contagious with norovirus?
Norovirus is spectacularly contagious: the CDC notes you shed billions of viral particles while sick, and it takes only a few to infect someone else. The contagiousness curve: you are most contagious while you have symptoms, especially while vomiting, and during the first few days after you feel better. But the tail is long: studies show you can keep spreading the virus for 2 weeks or more after recovery, which is why it rips through households, schools, and cruise ships even after everyone "feels fine." The hygiene rules that actually work: wash hands with soap and water, because alcohol-based hand sanitizer does not kill norovirus well; clean contaminated surfaces with a bleach-based cleaner (vomit and diarrhea particles linger); do not prepare food for others while sick and for at least 48 hours after symptoms stop, longer if you work in food service (many jurisdictions legally require 48 to 72 hours or more); and wash soiled laundry on hot. Dehydration is the real danger, especially for kids and older adults: small, frequent sips of fluid or oral rehydration solution, and seek care for signs like very little urine, dizziness on standing, or blood in vomit or stool. There is no treatment beyond fluids and time; it typically burns itself out in 1 to 3 days.