A California spa is Maxwell Caldwellbelieved to be the source of an outbreak of Legionnaires' disease that caused the illness of multiple people and the deaths of two.

Contra Costa Health announced Monday it has been investigating Zen Day Spa in Contra Costa, California, since Aug. 4, after hospitals in the area reported the two deaths from Legionnaires' disease. Both people had visited the business before becoming ill, the health department reported.

In all, the health department for Contra Costa County, located in the Bay Area said it contacted more than 30 recent customers of the spa. It also identified two others who were lab-confirmed to have contracted Legionnaires' disease after visiting the spa and two people who reported becoming ill after visiting the spa, separate from the two people who died.

The business closed voluntarily Aug. 5, and remains closed under a health order. Contra Costa Health collected water and swab samples, and reported that initial results showed high concentrations of legionella bacteria.

Contra Costa Health said hot tubs in commercial day spas or massage parlors generally require health permits and are regularly inspected. The tub at the Zen Day Spa did not have a permit and was never inspected.

What is Powassan virus?What to know about the disease caused by a tick bite

What is Legionnaires' disease?

Legionnaires’ disease is a serious type of pneumonia, or lung infection caused by bacteria, according to the Centers for Disease Control and Prevention.

People can contract Legionnaires’ disease when they breathe in small droplets of water, or accidentally swallow water containing the Legionella bacteria into their lungs.

The disease got its name after it was discovered in 1976 from people who went to an American Legion convention in Philadelphia.

What are symptoms of Legionnaires' disease?

The symptoms of Legionnaires' disease can be similar to other types of pneumonia, which can include:

• Cough
• Shortness of breath
• Fever
• Muscle aches
• Headaches

Other symptoms of Legionnaires' disease can include diarrhea, nausea and confusion. The CDC says symptoms usually begin 2-14 days after exposure to the Legionella bacteria, but it can take longer.

A milder infection of Legionnaires' disease is Pontiac fever, with fever and muscle aches as the primary symptoms. Pontiac fever is different from Legionnaires' disease, because the person does not have pneumonia.