The likelihood of developing legionellosis depends on the vulnerability of the individual and the route of exposure. The primary means of infection is through inhaling aerosols—tiny airborne particles that originate from droplets. These aerosols are typically produced when water is sprayed (e.g. from showers, irrigation systems) or splashes onto surfaces (e.g. turning on a tap or hosepipes spashing off surfaces). In healthcare environments, particularly in Care Quality Commission (CQC)-registered premises, aspiration—where water enters the respiratory tract instead of the digestive system during drinking—can also be a significant route of infection. Only one possible case of person-to-person transmission has ever been reported, and it occurred under exceptional circumstances.

Legionella bacteria vary in their ability to cause disease. Over 60 species have been identified, but only about half have been associated with human infection. The most common and virulent species is Legionella pneumophila, particularly serogroup 1, which accounts for most recorded cases and outbreaks. However, not all strains of L. pneumophila serogroup 1 are equally harmful—only those with specific virulence factors that allow them to enter human cells tend to cause disease. Strains reacting positively to monoclonal antibodies MAb2 from the “Joly” panel or MAb3/1 from the “Dresden” panel are most frequently implicated in outbreaks. Other serogroups such as 3, 5, and 6 have also been linked to illness. Additionally, non-pneumophila species such as L. longbeachae (commonly associated with potting soil and prevalent in Australia), L. anisa, and L. bozemanii have been known to cause infection, particularly in immunocompromised individuals.

The health risk from Legionella is influenced by several factors: the concentration of bacteria inhaled or aspirated, duration and timing of exposure, and the susceptibility of the person exposed. Men are about three times more likely to contract the disease than women. Risk increases with age, particularly over 40, and is further elevated in individuals who smoke or have underlying health conditions such as diabetes, heart or lung disease, or any condition—whether illness or treatment-induced—that weakens the immune system. It is known that even brief exposures can result in infection; for example, during the BBC outbreak, one individual became ill after merely passing through the Piccadilly Circus area on an open-top bus.

Both the extent of exposure and the vulnerability of the exposed individual must be taken into account when conducting a legionellosis risk assessment. This is a legal requirement for all employers, self-employed individuals, and anyone responsible for premises used in work-related activities.

The Health and Safety Executive (HSE) provides guidance that categorizes Legionella colonization into three levels: under control, requiring monitoring, and requiring urgent investigation and remedial action. Full details can be found in the HSE publication Legionnaires’ Disease: The Control of Legionella Bacteria in Water Systems (commonly referred to as “L8”). This document includes interpretation tables and recommended actions for systems such as cooling towers (page 31) and domestic hot and cold water systems (page 48). In high-risk settings, such as healthcare facilities, a Water Safety Group may determine that any detectable level of Legionella poses an unacceptable risk to certain patients.