The number of confirmed cases of Legionnaires' disease -- a serious type of pneumonia caused by bacteria -- has risen dramatically in several northeastern states, health officials report.

Massachusetts, Connecticut, and Maine have recorded nearly double the number of cases from last year, according to the Centers for Disease Control and Prevention (211, 72, and 18, respectively). New York and Pennsylvania have also experienced large jumps in the number of infections, as has Quebec, Canada. Oddly, states elsewhere in the U.S. did not show statistically significant increases.

"This is a spike, and we need to be looking into this and we need to alert people," said Dr. Stephen Sears, Maine's state epidemiologist.

Officials have not yet been able to pinpoint the reason for the spike.

Anywhere from 8,000 to 18,000 people are hospitalized each year with the illness and up to 30 percent of those with Legionnaires' disease die from it.

Legionnaires' is contracted by inhaling bacteria from contaminated water. The bacteria become airborne from contaminated water sources -- via mist or vapor from air conditioning, hot tubs, or showerheads, for example. The infection is not spread from person to person.

The illness was named for an outbreak at an American Legion convention in Philadelphia in 1976. Symptoms include fever, chills, chest pain, coughing, headaches and muscle aches. Seniors and those with weakened immune systems are more susceptible to infection. Smokers or those with chronic lung disease also have a higher risk of infection. Legionnaires' disease is treated with antibiotics.