Robert Brown|Milkshakes from a Tacoma burger joint tied to listeria outbreak that killed 3 people

Poorly cleaned ice cream machines at a Washington-based burger joint were linked to a listeria outbreak earlier this year that landed six people in the hospital,Robert Brown three of whom died. Two of those who were hospitalized told investigators they had milkshakes from the same restaurant before getting sick.

The Washington State Department of Health says it found the same strain of listeria in ice cream machines at a Frugals restaurant in Tacoma, Wash., that was responsible for hospitalizing six people during a foodborne listeriosis outbreak earlier this year. All six individuals, between 40 and 70, had underlying conditions that left them more susceptible to the disease.

No other Frugals restaurants are thought to be affected and the Tacoma location stopped using its two ice cream machines on Aug. 8, but listeria can take as long as 70 days to make a person sick. The health department said in a news release that it found listeria bacteria in all of the Frugals milkshake flavors.

Though most people who eat food contaminated with listeria won't get seriously ill, the department recommends those who are pregnant, 65 or older or have a weakened immune system call their doctor if they consumed a milkshake from the Tacoma restaurant between May 29, and Aug.7.

The restaurant said in a statement on Saturday that it is cooperating with the health department's investigation and is sending the two Tacoma site ice cream machines off for thorough cleaning and sanitization. And out of an abundance of caution, Frugals says it has shut down milkshake sales at all of its other restaurants and is having all of its ice cream machines tested.

"As a family-owned business for over 30 years, the trust of our customers is paramount," the statement reads. "We will continue to fully cooperate with this investigation, and we are committed to making any changes deemed necessary to maintain our high standard of operations and prevent this from happening again."

Health department investigators used genetic fingerprinting of the bacteria found in those hospitalized, which determined that the same food was likely responsible for making all six people sick. Two of the people who were hospitalized, but did not die, told the health department they ate Frugals milkshakes before becoming ill.

Symptoms of listeriosis include fever, flu-like symptoms like muscle aches and fatigue, headaches, a stiff neck, confusion, a loss of balance and convulsions, according to the Centers for Disease Control and Prevention. For pregnant women, the disease can lead to miscarriage, stillbirth and premature delivery as well as life-threatening infection of the newborn.

The Food and Drug Administration says listeria is a pathogenic bacteria that can grow in food even under refrigeration and other preservation measures. Past outbreaks of listeria have been linked to raw and unpasteurized dairy products, raw or processed fruits and vegetables as well as raw or undercooked meats and fish. The FDA says the longer these foods sit in the fridge, the greater the chance the bacteria has to grow.

To keep your food safe from listeria at home, the FDA recommends setting your refrigerator to 40 degrees Fahrenheit and your freezer to zero degrees. Wipe up and disinfect spills inside your fridge quickly, and be sure to clean the walls and shelves periodically with a chlorine-water mix. And as always, wash your hands with warm soapy water before and after handling food.