With two million cases in the UK every year, norovirus exceeds all other causes of diarrhoea and vomiting by many orders of magnitude. If a malevolent person had set out to create it as the nastiest virus known, their only disappointment would be a lethality failure; the vast majority of victims get better after two or three days without specific treatment. Just as well, because there isn’t one.
But in everything else the virus is a success. Only a tiny number of virus particles need to be ingested to start an infection. They are very tough, can survive on a contaminated surface for weeks, and are not killed by the usual mild kinds of domestic disinfectant. The immunity given by an infection is short lived. And the virus’s speciality is projectile vomiting, often without warning. As the virus is in the vomit, the hit rate in a crowded room is high. If a chef is caught short in the kitchen and is sick into the sink, exposed food will be contaminated. Aerosols from violent vomiting travel. The virus is in the diarrhoea as well. That is why oysters are high risk. Human sewage is a delicacy for them. As filter feeders they concentrate the virus in their flesh. Eat them cooked.
Outbreaks on cruise ships and in hotels are notorious. Eight prison outbreaks have been recorded so far this season. Norovirus is a nightmare for hospitals. It spreads easily in the best-run wards. The only response is to isolate the infected, close the ward and carry out a deep clean. In the current norovirus season (which runs from July to June to include the winter peak in a single year) 291 closures have been recorded in England and Wales. Last season there were 1291. This year norovirus started early and with vigour. Nobody knows why. A vaccine is years away, if ever.