From all the media coverage one would think Norwalk virus was something new and terrible, but it's neither. One of the more common causes of viral gastroenteritis or the vomiting and diarrhea we sometimes call "stomach flu", Norwalk-like viruses have probably been with us a very long time. Because it's so small we were only able to identify it and the other common viruses that cause viral gastroenteritis since the early 1970's.


These sinister little beasts are not really living, as they are simply tiny chains of molecules, containing biological code, wrapped in a protein envelope called a capsid. Being so small, they easily penetrate our bodies and enter cells taking over control of cellular activity. The viral code is contained in DNA or RNA strands that alter the cell's normal instruction codes. It converts the cell into a factory producing virus particles identical to itself, until it kills the cell. Just like computer viruses, once it has established itself, until it is successfully eradicated, it continues to replicate, spreading to other parts of the body and exporting itself to other susceptible hosts.

Viruses can cause a wide range of infections ranging from warts and the common cold to polio, mumps, measles, hepatitis, meningitis, pneumonia, AIDS and Ebola hemorrhagic fever. Because viruses alter the controlling codes in our DNA or RNA, some types of virus are able to remove the normal stop switch on cell division, resulting in uncontrolled abnormal cellular replication; in other words, cancer. Some types of lymphoma, cervical carcinoma, and probably more cancers than we currently know, are caused by previous virus infections.


A member of the calicivirus family, a group of small RNA viruses that have a wide variety of sequences of nucleic acids in their RNA, it accounts for up to 80% of epidemics of gastroenteritis seen in nursing homes. It is spread from the digestive tracts of infected humans via fecal contamination and oral ingestion, to other susceptible humans. Once in the bowel the virus infects the cells lining the small intestine. After a brief period of rapid viral replication within these cells, they are destroyed, allowing fluids and salts to leak from the body into the gut, causing viral-laden watery diarrhea.

Epidemics have been traced to oysters harvested from beds near sites of improper sewage disposal, contaminated sources of drinking water, fresh fruit and vegetables irrigated with contaminated water and infected food handlers. Water droplets containing virus or secretions on objects people touch, such as door handles can also spread the virus.

Norwalk virus is easily spread during epidemics for several reasons:

  • It only takes a very few virus particles to cause the infection in susceptible hosts
  • People with infections shed the virus for up to 2 weeks even after they have recovered from their symptoms
  • The virus survives freezing, heating to 60º C and up to 10 ppm chlorine
  • There are many varieties of the virus strain and immunity is specific for only a single strain
  • Even after infection and development of immunity, that immunity fades with time, rendering the person susceptible to further infection

Following exposure to the virus, after an incubation period of one to three days, the infected person experiences fever, aches, headache, vomiting and diarrhea with cramps. Younger children are more likely to only experience vomiting while in adults, diarrhea is more common. The diagnosis is made based on the clinical presentation and the presence of other similarly infected contacts rather than on lab tests, as these are difficult and expensive, usually done in research laboratories. In all but the very sick, or people with impaired immunity, the body mounts a successful immunological defense by producing a specific antibody and the sufferer recovers in 2 or 3 days. If vomiting is severe and protracted or if serious dehydration is suspected then intravenous replacement of electrolytes and fluids is necessary.


Because Norwalk is one of the more contagious viruses, if you are in close contact with infected people and you are susceptible, you will likely get sick. That's why, once it is established in dormitories, old folks homes, elementary schools and cruise ships, it will sweep through the confined population causing a dramatic epidemic. Careful hand washing, in which each finger is carefully scrubbed using soap and abundant water followed by rinsing, is effective in removing the virus. Contaminated materials and surfaces should be heat sterilized or cleaned using 70% alcohol or 10% bleach.

Although news media have reported epidemics of this terrible new virus, we should probably put it in perspective. It's neither new nor uncommon. As virus infections go, Norwalk-like virus infection, although inconvenient, uncomfortable and perhaps embarrassing, is not particularly serious. If we want to worry about our health we could choose scary, dangerous things - like smoking, excessive drinking or riding in cars.

Dr. Ray Baker is Assistant Clinical Professor in the Faculty of Medicine at the University of British Columbia. He has been awarded fellowships in both Family Medicine and Addiction Medicine. He has been a practicing physician for over 23 years. From 1993 to 1997 he represented Canada on the Board of Directors of the American Society of Addiction Medicine, North America's credentialing body in this specialized area of medicine. His area of special clinical expertise is in assessment and treatment planning of the worker disabled by one of the "invisible disabilities", stress, depression, chronic pain syndrome or substance use disorder.