Many more people may be sickened in the outbreak than the current numbers indicate. For many infected people, the bacteria only cause relatively mild infections that don't require medical care. Even if they do seek medical care, laboratories don't routinely test for listeriosis in patients with mild gastrointestinal illnesses. Lastly, while many infected people develop symptoms within two weeks of an exposure, symptoms can develop up to 10 weeks after eating a contaminated food, and there are additional lag times in connecting cases to an outbreak.