Health department investigators visited the store's deli four times over the course of about six weeks starting in early August this year, each time recording "inconsistent" hand washing and "improper" cleaning of surfaces that touched food.

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King County health officials are trying to figure out the source of salmonella that has made seven people sick after tracing the bacteria to food prepared in a Costco deli.

The people had fallen ill “sporadically” over a period of nearly 11 months, starting in August 2017, the county’s health department said Wednesday in a statement posted on its website. All suffered from an identical genetic strain of salmonella, “suggesting a common source of infection,” the statement said. They’ve all since recovered.

All seven people had shopped at Costco’s Issaquah store, including five who bought ready-to-eat food from the deli. One person’s food-purchase history couldn’t be determined, while another infected person was an employee at that deli. There’s no evidence that the employee was the source, the statement said.

Health department investigators visited the deli four times over the course of about six weeks starting in early August this year, each time recording “inconsistent” hand washing and “improper” cleaning of surfaces that touched food. On Sept. 19, the deli was ordered to stop serving ready-to-eat foods until it improved its practices. Two days later, officials were satisfied with a cleaning that was performed by the store and by its plan to further train employees, the health department said.

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About 1.2 million people in the U.S. suffer from salmonella-related sickness every year, the vast majority of which is caused by food, according to an estimate by the Centers for Disease Control and Prevention. Symptoms include fever, stomach cramps and diarrhea, which is sometimes so severe that victims need to be hospitalized. Those symptoms usually last for four to seven days, but sometimes the bacteria can prove deadly. It kills about 450 people a year in the U.S., CDC data shows.

Costco said in an email Wednesday that all employees have been retrained in standard sanitation procedures and that all of their departments are monitored.

“We have worked closely with the local health department to provide them with any data that they may need during their investigation and have followed their advice and direction during the process,” the statement said.