(CNN) Nine former guests at a prominent Atlanta hotel have now been diagnosed with Legionnaires' disease, but medical investigators have not yet found the exact source of the bacterial infection, officials said.

"Based on epidemiological evidence we have an outbreak among people who stayed at the [Sheraton Atlanta] during the same time period," said Nancy Nydam, director of communications at Georgia Department of Public Health.

Nydam said Tuesday that all nine cases of Legionnaires' disease have been confirmed by testing. No deaths have been reported.

Legionnaires' is a serious form of pneumonia that is noncontagious. Guests who complained of lung problems and were later diagnosed with Legionnaires' had attended a convention at the Atlanta hotel a couple of weeks ago.

The bacterium causing Legionnaires' has not been confirmed at the hotel, which has hired outside experts to conduct testing.

Nydam said. More than 400 guests have been relocated to nearby hotels, The hotel has voluntarily closed down until the source is found and the problem is fixed,Nydam said. More than 400 guests have been relocated to nearby hotels, CNN affiliate WSB-TV reported last week.

Thousands infected each year

About one in 10 people who get sick from Legionnaires' disease will die, a recent government report found.

The disease infects an estimated 10,000 to 18,000 people in the United States each year. People can get sick when they breathe in mist or accidentally take water into their lungs containing the bacteria. It can be treated with antibiotics, according to the US Centers for Disease Control and Prevention.

The hotel's general manager, Ken Peduzzi, said that immediately after being notified of a possible bacterial contamination, "the hotel retained outside environmental consultants and is presently collecting samples." The state health department, the Fulton County Board of Health and environmental specialists are working with the hotel to test for the bacteria.

"At this time it remains unknown if the source of the exposure is located within the hotel. Samples will be collected from various areas of the hotel, including the pool, hot tub, fountain, and chillers," said Peduzzi on Friday.

"This is the typical way these situations are handled since the assessment and testing can be complicated," according to Nydam. The state health department and other agencies will work with them on the next steps in the investigation (technical assessment, sampling plan and submission)," she added.

In addition to relocating current guests to nearby hotels, the Sheraton is also reaching out to guests with upcoming reservations, according to Peduzzi. "All guests with upcoming reservations through August 11th have been advised of the hotel's temporary closure and are working with Marriott and Sheraton Atlanta associates to find alternative accommodations. Guests whose reservations have been canceled will receive full refunds," he said.

James Francey, one of more than 400 relocated guests, told WSB: "This a hazard of travel ... so OK it happens. The CDC is here in town, so that's great."

Peduzzi noted that "many hotel employees have already been redeployed to other hotel properties or continue to work in off-site locations. We are hoping to limit work disruption while the hotel is closed and keep Sheraton Atlanta's associates gainfully employed."

Symptoms of Legionnaires' disease

Legionnaires' begins with a patient feeling tired and weak, according to the educational organization Legionella.org . Other common symptoms include coughing, diarrhea, nausea, vomiting, headaches, muscle aches, chest pain and shortness of breath. The incubation period -- the time it takes for symptoms to appear after a person is infected with the bacteria causing the disease -- is from 2 to 10 days.

Described as a "severe, often lethal, form of pneumonia," Legionnaires' can lead to treatment in an intensive care unit, according to Legionella.org. Some symptoms may be long-term: One study showed that three quarters of survivors continued to feel tired, 66% had neurologic symptoms and 63% had neuromuscular symptoms months after their diagnosis.

Scientists dubbed the illness "Legionnaires' disease" following an outbreak in Philadelphia in 1976, largely among people attending a state convention of the American Legion, according to the CDC. Subsequently, the bacterium causing the illness was named Legionella pneumophila.