Over 100 cases of possible norovirus have been reported in Kumamoto Prefecture, the prefectural government said Tuesday, with a local river identified as a potential culprit.

On Aug. 13, seven high school students who were playing near Todoroki Falls in the city of Amakusa experienced vomiting and diarrhea after playing in the river nearby, and similar cases were reported in the following days. All reported symptoms were mild, according to the city.

The city has since then set up signs near the river, calling on visitors to refrain from playing in the water.

A water quality inspection by the prefecture has detected norovirus in the water samples retrieved from the waterfall¡¯s plunge pool and upstream. The prefecture and medical institutions also inspected stool samples from 16 patients, and detected norovirus in six of them.

The prefecture, however, said it is too soon to reach a conclusion, as the genetic type of the virus found in the water was different to those found in the stool samples.

Norovirus is a highly contagious virus that causes vomiting and diarrhea, and is mostly spread through oral transmission. People can get infected by having direct contact with an infected person, eating or drinking contaminated food or liquids, or putting unwashed fingers in the mouth after touching contaminated objects or surfaces, according to the U.S. Centers for Disease Control and Prevention.

As of Wednesday, a total of 124 people have sought medical attention due to related illness, according to Kumamoto Prefecture.

Earlier this month, the city of Yufu in Oita Prefecture also reported an outbreak of norovirus-related food poisoning that might have been caused by drinking contaminated water in a local Japanese-style hotel. A total of 537 people in 24 prefectures had complained of symptoms as of Aug. 19, NHK reported.

Japan saw 163 outbreaks of norovirus-related food poisoning in 2023, with 5,502 seeking medical treatment, according to the health ministry.