For the less severe, non-invasive listeriosis, mild symptoms such as fever, muscle aches, nausea, vomiting, and diarrhea may occur, and often persists for 1 to 3 days.
For the more severe, life-threatening invasive form of the disease, symptoms may include headache, stiff neck, confusion, loss of balance, and convulsions. Invasive listeriosis is a potentially life-threatening event, especially for newborns, adults over the age of 65, and those with weakened immune systems.