FILE - This Thursday, Jan. 12, 2017, file photo shows a sign on a Chipotle restaurant. A person who reported eating at a Chipotle in northern Virginia has tested positive for norovirus. On Thursday, July 20, 2017, health officials said that's not yet enough to determine the cause of the roughly 60 reported illnesses it has identified. (AP Photo/Gene J. Puskar, File)

FILE - This Thursday, Jan. 12, 2017, file photo shows a sign on a Chipotle restaurant. A person who reported eating at a Chipotle in northern Virginia has tested positive for norovirus. On Thursday, July 20, 2017, health officials said that's not yet enough to determine the cause of the roughly 60 reported illnesses it has identified. (AP Photo/Gene J. Puskar, File)

NEW YORK (AP) — A person who reported eating at a Chipotle in northern Virginia has tested positive for norovirus. But health officials say that’s not yet enough to determine the cause of the roughly 60 reported illnesses it has identified.

The location in Sterling, Virginia, was temporarily closed for cleaning this week after customers reported symptoms that Chipotle said were consistent with norovirus. That sent shares of the restaurant chain down, underscoring how vulnerable its reputation remains almost two years after an E. coli outbreak that sent its sales plunging.

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Norovirus is a leading cause of illnesses from contaminated food, and infected employees are a frequent source of the outbreaks. Symptoms include vomiting, diarrhea and nausea. Overall, one out of six Americans get sick each year by consuming contaminated food or drinks, according to the Centers for Disease Control and Prevention.

The Loudoun County Health Department in Virginia, outside Washington, D.C., says it is still awaiting further test results that should be available early next week. David Goodfriend, director of the health department, says that having another person test positive for norovirus would be a “very strong indication” that norovirus was behind the illnesses. The health department says it has identified about 60 sickened people who said they ate at the Chipotle in question.

Chipotle Mexican Grill Inc. has stressed the safety measures it has taken to prevent such occurrences or respond quickly when they do happen. But its image is still fragile, and the chain was dealing with more bad publicity Thursday after mice fell from the ceiling in a Dallas location. Chipotle says it was an “extremely isolated incident,” and that the mice got in the restaurant because of a structural gap in the building. It said the gap has been fixed.

Shares in Chipotle fell 4.5 percent to close at $356.05, their lowest point since April 2013. The shares had been above $390 on Tuesday.