When talking about the bugs that cause foodborne illness most people focus on familiar culprits like E. coli O157:H7, Salmonella, Campylobacter, Listeria, and Staph aureus. But the real heavyweight of foodborne illness germs is not a bacterium at all. It is a virus and it is far and away the most common cause of foodborne illness outbreaks. According to the most recent data from CDC, among 1,270 outbreaks in 2006, 621 had a confirmed single cause, and that cause was most often norovirus (54 percent of outbreaks), followed by Salmonella (18 percent of outbreaks). It is a wonder that norovirus gets so little attention–unless of course you are on one of those cruise ships that seem to suffer norovirus outbreaks regularly.
Norovirus is such a common problem precisely because it is a virus. Viruses are small, really small. Viruses range from 20 to 250 nanometers (one nanometer is a billionth of a meter). By comparison, bacteria are more on the order of 1,000 nanometers. If a bacterium were the size of a human, a virus particle would be a mouse. Mind you, bacteria are plenty small themselves. We have about 10 times as many bacteria in our bodies and we have cells. You could fit hundreds of thousands of bacteria on the head of a pin.
And here is the bad news: it takes only 10 to 100 norovirus particles to cause an infection. But because those particles are so infinitesimal, a single vomiting episode can expel over 30 million particles. But the bad news does not end there. Two other factors help make norovirus the most common cause of foodborne illness: unlike bacterial gastroenteritis in which the infectious bacteria is contained only in the feces, norovirus is present in both the vomitus and feces of an infected person; further, because it is so tiny, norovirus can aerosolize. If an infected person vomits, norovirus is in the air. Flush the toilet and you cause aerosol generation. Those aerosolized viral particles can then settle on any surface ready to be picked up by the next pair of hands.
And that is another important point about norovirus as compared to bacterial food pathogens. As the CDC reports, foodborne outbreaks of norovirus occur most often when infected food handlers fail to wash their hands properly after using the toilet. With bacterial pathogens, the contamination typically occurs independently from food handlers.
Symptoms of a norovirus infection can occur as early as 12 hours after exposure, but typically within 24 to 48 hours after the virus is ingested. Noroviruses cause the classic “stomach flu.” Real influenza, such as H1N1, is an upper respiratory illness. Symptoms of a norovirus infection typically begin abruptly and include nausea, vomiting, diarrhea and stomach cramps. A low-grade fever, chills, headache, and muscle aches may also occur. Vomiting is often relentless during the first 24 hours. But if there is any good news about norovirus infections, they are usually relatively short-lived and a full recovery generally occurs after a couple of days.
As with most viral illnesses, there is not much to do about a norovirus infection except suffer through it. A visit to the doctor during the acute illness is only apt to risk infection of others. Occasionally, a norovirus infection can cause a very serious illness, typically in the elderly or those with underlying health problems. The most likely complication is volume depletion–dehydration–particularly since rehydration is most difficult while vomiting continues. In cases of serious volume depletion a trip to the ER and an IV may provide the only relief.
If you suspect you have suffered a norovirus infection–stay home! Make sure you use a disinfectant in your bathroom, wash your hands ruthlessly, and try to limit contact with other people until at least three days after recovery.