Outbreaks of norovirus -- which causes infectious stomach and intestinal ailments -- have prompted Prime Minister Shinzo Abe to call on the health ministry to take special measures. The ministry's statistics show that from Nov. 1 to Dec. 18, a record 9,650 people suffered from food poisoning believed to have been caused by the virus -- about 5.5 times more than the number reported a year ago. The figure does not cover all cases. Several deaths possibly caused by the virus have also been reported. It is not known why norovirus is rampant this year.

Symptoms from norovirus infection include vomiting, diarrhea and fever. The incubation period is one to three days. Although symptoms usually disappear in one to three days, elderly people must take precautions. The infection can kill as a result of severe dehydration due to vomiting or diarrhea. In 2005, norovirus was responsible for 18 percent of food-poisoning outbreaks reported to the health ministry. The largest percentage of all people hit by food poisoning (32 percent) were infected with the virus.

There are two routes of norovirus infection. One is through eating food; shellfish such as oysters tend to carry the virus. The other route is human-to-human transmission. More than half the reported cases occur as a result of the latter. Fecal material and vomit from a patient contain a large amount of norovirus. When they become dry, the virus may become airborne and infect others.