Japanese health officials are reporting an increase in norovirus infections, in fact, it’s at the highest levels in a decade, according to the National Institute of Infectious Diseases.

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The number of patients reported by some 3,000 pediatrics departments across Japan averaged 20.89 during the week to Dec. 18, the highest for a mid-December period since 22.81 in 2006.

Nearly half of Japan’s prefectures are at alert level. Yamagata prefecture has reported the largest patient number per institution (47.27), followed by Miyagi, Saitama and Miyazaki.

Norovirus is a highly contagious viral illness that often goes by other names, such as viral gastroenteritisstomach flu, and food poisoning.

The symptoms include nausea,vomiting, diarrhea, and some stomach cramping. Sometimes people additionally have a low-grade fever,chills,headache, muscle aches, and a general sense of tiredness. The illness often begins suddenly, and the infected person may feel very sick. In most people, the illness is self-limiting with symptoms lasting for about 1 or 2 days.

In general, children experience more vomiting than adults do.

Norovirus is spread person to person particularly in crowded, closed places. Norovirus is typically spread through contaminated food and water, touching surfaces or objects contaminated with norovirus and then putting your hand or fingers in your mouth and close contact with someone who is vomiting or has diarrhea.