The Centers for Disease Control and Prevention has not identified a food that might be spreading the deadly bacteria, but officials said the public should be alert to symptoms and the possibility of infection.
Those symptoms include fever, muscle aches, nausea, and diarrhea.
Listeria can be treated with antibiotics, but it is especially dangerous to pregnant women, newborns, the elderly, and those with compromised immune systems.
CDC officials say nearly all the 23 people known to have been infected in the outbreak either live in, or traveled to, Florida about a month before they got sick.
Listeria is one of the most dangerous forms of food poisoning, and 22 of the infected people were hospitalized. One person from Illinois died and one pregnant woman lost her fetus, the CDC said.
Listeria symptoms usually start one to four weeks after eating contaminated food, but can start as soon as the same day.
The first cases occurred in January of this year, but have continued through June, when two of the people got sick, CDC officials said.
By Mike Stobbe