Tacoma, Washington - Three people have died after being infected with Listeria bacteria traced to a Tacoma, Washington, restaurant's ice cream machine, health officials said.

Listeria was present in all three milkshake flavors sold at Frugals restaurant, the Washington State Department of Public Health said Friday. Six people were hospitalized in total after consuming milkshakes there.

"Investigators found Listeria in the ice cream machines, which were not cleaned correctly," the department said in a statement. "No other Frugals restaurants are believed to be affected. The restaurant stopped using its ice cream machines August 8, but Listeria can sicken people up to 70 days later."

Public health experts tested the genetic fingerprint of the milkshake bacteria and matched it to the Listeria strain that had sickened the six people between February 27 and July 22.

Listeria-contaminated food does not cause serious illness in everyone, but people who are pregnant, who are aged 65 and up, or whose immune systems are weakened could be vulnerable to harsher illness, the department warned.

All six people who got sick had health conditions that compromised their immune systems' ability to fight off infection, the health department said.

Officials urged anyone in the vulnerable category to check in with their doctor if they felt sick after downing a milkshake from the Tacoma restaurant in recent weeks. Once detected, Listeria can be treated with antibiotics.

Symptoms to watch for include fever, muscle aches, and tiredness in people who are not pregnant. It also may trigger a headache, stiff neck, confusion, loss of balance, or convulsions.

In a pregnant person, Listeria can cause a mother to miscarry, go into premature labor, or birth a stillborn baby, according to the US Centers for Disease Control and Prevention. It also can cause "serious illness and even death in newborns," the CDC said.

On top of that, pregnant women are 10 times more likely to get a Listeria infection than anyone else.

Listeria is the third leading cause of food-poisoning deaths in the US, with about 1,600 people getting sick and 260 dying annually, the CDC said.