According to MedicineNet, people are most likely to pass on the Norwalk virus while they are experiencing symptoms, but they are somewhat contagious for up to two days beforehand and may be for several weeks afterward. The length of time is mostly determined by a person's immunity.
Norwalk viruses are a common cause of diarrhea and are spread in the stool. Poor hygiene is identified by Mayo Clinic as one of the major causes of transmission of the virus, especially where large groups of people are kept together such as in hospitals and nursing homes. Food poisoning, particularly from raw produce and shellfish, and contaminated water are also sources of the virus. The time between acquisition of the virus and the appearance of symptoms is typically between 12 and 48 hours.