New Italian research is drawing renewed attention to Legionella as autumn rains arrive, with studies examining how weather may relate to the bacterium that can multiply in water systems and reach the lungs through tiny contaminated droplets. National surveillance data recorded an 18% increase in cases last year.
Two Italian studies, one published on October 16, 2026 in BMC Public Health and another in March 2026 in Eurosurveillance, examined the relationship between rainfall and reported Legionella infections. These findings, based on 1,932 cases in the Lazio region and 27,458 national cases respectively, point to a complex relationship between weather patterns and the course of cases that suggests integrating meteorological data with health surveillance.
How Rainfall Relates to a Bacterial Threat
The connection between autumn weather and Legionella is not direct. The bacterium can find favorable conditions in artificial water systems: stagnant water, suitable temperatures, encrustations and biofilms—microbial communities that adhere to the inner surfaces of pipes.
Once established in these environments, the bacterium can be dispersed into the air in droplets from sources like showers, whirlpool baths, fountains, and cooling towers. Respiratory exposure to these aerosols is the main route of infection. Being out in the rain does not equate to contracting legionellosis; and the presence of the microorganism in the environment does not automatically mean disease—the quantity of bacteria, the mode of exposure, and the condition of the person all come into play.
The BMC Public Health study, by Spaziante M. and colleagues, examined 1,932 cases of legionellosis reported in the Lazio region between 2015 and 2023, analyzing personal characteristics, territory, and meteorology together. The researchers found that average monthly precipitation was associated with a higher incidence, alongside factors like advanced age, male sex, and the presence of other diseases. The study noted that this statistical association does not prove that every rainstorm causes infections; risk is multifactorial, involving both the environment and the vulnerability of those exposed.
Mapping the Lag Between Weather and Illness
The Eurosurveillance study, by Sciurti A. and colleagues, provides a more granular picture of the timing involved. By analyzing 27,458 community-acquired cases in Italy from 2005 to 2023, the researchers identified a seasonal pattern with an autumn peak. They found that the strongest associations between temperature and humidity and disease incidence concerned conditions observed nine to ten weeks before the onset of illness, while for precipitation the interval was mainly one week.
According to the authors, these windows describe relationships between the environment and the course of cases, not the incubation period of the individual patient, and weather may influence bacterial proliferation and exposure opportunities at different times. They suggest integrating weather data with health surveillance, while noting that the results do not allow attributing a precise share of infections to climate change or predicting who will fall ill after a storm. The 2024 data from Italy’s Istituto Superiore di Sanità, which recorded 4,627 cases—an 18% increase from 2023—illustrates the scale of the problem, with over three-quarters of cases occurring in people aged at least sixty.
Preventing Legionella in Water Systems
- Autumn rain is associated with Legionella risk through the environment and water systems, not by spreading the bacteria directly through rainwater itself.
- The studies identified lag times between weather conditions and case onset, describing relationships between environment and case trends rather than the incubation period of individual patients.
- Prevention depends on the management of water in buildings, including reducing stagnation and properly managing temperatures.
From Diagnosis to Treatment: The Clinical Response
Fever, cough, chest pain, and difficulty breathing can be accompanied by diarrhea or confusion. These manifestations can appear in other infections as well: Legionella cannot be recognized from symptoms alone, and even a chest X-ray does not distinguish it with certainty from other causes of pneumonia. Medical evaluation is needed and, when indicated, a specific search for the bacterium.
The CDC recommends pairing the urine antigen test with tests on respiratory samples, because the urinary test generally identifies only one serogroup of Legionella pneumophila. Reporting recent hotel stays or hospital admissions can help doctors direct diagnostic work-up. Significant breathing difficulty requires urgent care. Antibiotic therapy should be chosen by a doctor and not delayed while awaiting test results.
Managing Water Systems for Prevention
The CDC provides guidance on public health strategies for Legionella control, emphasizing the need for water management in buildings to regularly monitor and maintain water systems.
The core of prevention lies in the meticulous management of artificial water systems. Hospitals, nursing homes, hotels, and apartment buildings have different needs but share a common requirement: maintenance must be scheduled and verifiable. This involves reducing stagnation, properly managing temperatures, and cleaning and, when necessary, disinfecting systems. The question for public health systems is not just who performs these checks, but how their effectiveness is verified and how interventions are triggered when a potential problem is identified.
Seeking Medical Help for Pneumonia Symptoms
However, certain groups deserve particular attention:
Older adults. Smokers. People with chronic medical conditions. Individuals with weakened immune systems. Anyone experiencing pneumonia symptoms—fever, cough, chest pain, or difficulty breathing—should seek medical evaluation promptly. If symptoms include confusion or significant respiratory distress, immediate medical attention is critical. Informing your doctor of any recent stays in hotels or hospitals is important for accurate diagnosis.
The Data on Risk: An Epidemiological Overview
The following table summarizes key demographic and temporal data from recent Italian surveillance studies, highlighting the populations most affected by the seasonal rise in Legionnaires’ disease.
| Parameter | Statistic | Source |
|---|---|---|
| Total National Cases (2024) | 4,627 | Istituto Superiore di Sanità (2025) |
| Annual Increase (2024 vs. 2023) | 18% | Istituto Superiore di Sanità (2025) |
| Total Community-Acquired Cases (2005-2023) | 27,458 | Eurosurveillance (2026) |
The Path Forward: Integrating Health and Climate Data
The convergence of these studies points to a clear conclusion: public health must adapt to the influence of environmental factors on disease. The concept of “climate health” is not abstract; it is the ability to turn weather information into a more attentive health organization. For healthcare systems, this means using weather data to orient surveillance and focusing on the management of water in buildings.
The research from Lazio and the national data provide a foundation for this shift. They indicate that while it is not possible to attribute a precise share of infections to climate change or to predict which individual will fall ill after a storm, it is possible to identify relationships between environmental conditions and case trends. This knowledge supports a more focused allocation of resources, centered on the maintenance of building water systems and attention to the most vulnerable people.
Linking environmental data with clinical reporting can make it faster to identify a problem. For those who live in or are treated in a facility, this means protection before the emergency. By linking environmental data with clinical surveillance, health authorities can identify problems faster and implement interventions sooner.
References
- Spaziante M, et al. Unraveling legionellosis dynamics in the Lazio Region: effects of individual, geographical and meteorological determinants. BMC Public Health. 2026 Oct 16.
- Sciurti A, et al. Seasonality and effects of climatic exposures on community-acquired Legionnaires’ disease incidence, Italy, 2005 to 2023. Eurosurveillance. 2026;31(12).
- Bellini B, et al. I casi di legionellosi segnalati al sistema di sorveglianza nazionale. Italia, 2024. Bollettino Epidemiologico Nazionale. 2025;6(2):9–18.
- Centers for Disease Control and Prevention (CDC). Laboratory Testing for Legionella. Last updated May 13, 2026.
- Centers for Disease Control and Prevention (CDC). Public Health Strategies for Legionella Control. Last updated March 26, 2026.
