Colorado Local health officials said they hope a norovirus outbreak in Buena Vista that left approximately 20 people ill in mid-November will serve as a reminder to area restaurants and food providers to be extra cautious.

Chaffee County Public Health received complaints from five people saying they were ill after eating food from the Subway in Buena Vista, according to a restaurant inspection report.

Lab tests on samples from the five individuals all tested positive for norovirus — an extremely contagious virus that can cause stomach pain, nausea, diarrhea and vomiting, according to the Centers for Disease Control and Prevention (CDC).

Public Health estimates roughly 20 people, mostly students at Buena Vista High School, might have fallen ill in the outbreak.

An investigation by Chaffee County Environmental Health Manager Victor Crocco, who declined to name the restaurant, found that one employee reported feeling ill the day after the illness was originally reported.

The restaurant was cited for three noncritical items, according to the inspection report, but the items cited likely did not lead to the outbreak, Crocco said.

Norovirus is the leading cause of illness and outbreaks from contaminated food in the U.S., according to the CDC. While an infected person can shed billions of norovirus particles, it only takes contact with as few as 18 particles to become infected.