Seasonality, local weather and infectious disease: effects of heat and humidity on local risk for urinary tract infections and Legionella pneumonia

Seasonality, or a cycling of high and low incidence, of infectious diseases has long been recognized but remains little understood. For many diseases, even major ones such as influenza, our knowledge of the seasonal drivers is very limited. One proposed driver of seasonality for many diseases is weather, especially temperature and humidity. I studied how likely an admission to a hospital was to be diagnosed with a UTI or pneumonia caused by Legionella across the US under various climates and weather conditions. I found that patients were 10–20% more likely to have a UTI when the monthly mean temperature was between 65–85°F compared to under 40°F. This may be due to slightly lower levels of hydration at warm temperatures reducing protection against UTIs. Pneumonia caused by Legionella was more common in warm (60–80°F) months than in cool or hot months. Within warm months, when humidity was above 60% there was a doubling in the odds of Legionella pneumonia. When the humidity was above 65%, the odds were quadrupled. Understanding why some diseases are seasonal and what role weather plays in this seasonality is important for both daily practice (e.g., recent weather can help diagnosis Legionella versus a more typical cause of pneumonia) and for larger policy adapting to changing weather and climate.