How common is it for somebody to get legionellosis?

Uncommon, not rare. Legionnaires disease was discovered from an outbreak of pneumonia at a hotel in philly of veterans. The bacterium was found in the air-condition/heating ducts. Old, uncleaned, poorly filtered ducts are still a common source of infection by inhaling the organism. Fortunately, it is quite treatable. Symptoms would be the same as any pneumonia. Fever, shaking chills, prouductive cough, chest pain...
CDC estimates. 8000 to 18000 get hospitalized for legionella in the US each year. Actual numbers are likely higher due to mild cases and unrecognized cases.