For many people exposure to Legionella will have no effect; however, for others it may cause mild symptoms, such as headache and fever. A small percentage of people exposed will become seriously ill. The elderly, smokers and persons with chronic lung disease are at higher risk for infection. Also at increased risk are immunocompromised individuals such as those with cancer, diabetes, or AIDS. Symptoms of the disease will usually appear 2 to 10 days after exposure. The annual incidence of Legionellosis nationally is between 20 and 25 cases per million people since 2015. More regionally, in San Francisco, it is between 1 and 5 cases per million people since 2015.