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What is the infectious dose of Legionella causing Legionnaires Disease?

John A. Herbert
John Herbert
Legionella Risk Assessment

Legionnaires' disease was identified in 1997, the potentially fatal illness is caused by the Legionella species. It occurs naturally, at low concentration, in groundwater, rivers, lakes, and streams.

It can survive utility water treatment, and given the right conditions, it may breed and multiply in building water systems.

The infectious dose is still unknown. However, one case where the source of a Legionella outbreak was traced to a source approx. six (6) km away, indicating that the Legionella infectious dose might be as little as 1 cfu.

legionella infectious dose

Since the infectious dose is believed to be very low, precautions are required, and water testing alone is deemed inadequate by some authorities.

John A. Herbert, legionella conference
centre: John Herbert, Legionella Conference 2003 Hong Kong

The solution is conducting a risk assessment, in some countries it is mandatory, to assess the risk in your circumstances.

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