The seasonal cycle of respiratory viral diseases has been recognized for thousands of years, with annual epidemics of the common cold and influenza occurring in winter in temperate regions. Epidemics caused by viruses such as SARS-CoV and SARS-CoV-2 also occur in winter. The mechanisms behind the seasonal nature of respiratory viral infections involve environmental factors and human behavior. Studies show that temperature and humidity affect the stability and transmission of respiratory viruses. Recent research highlights the importance of environmental factors in modulating host immune responses. This review discusses how outdoor and indoor climates influence transmission and host airway responses to viruses, and how these changes affect the seasonal circulation of respiratory viruses. Environmental factors such as temperature and humidity significantly influence the transmission of respiratory viruses. Low absolute humidity (AH) in winter leads to low indoor relative humidity (RH), which can enhance the survival and transmission of viruses. Studies show that respiratory viruses are more stable and transmissible under low RH conditions. Additionally, low humidity impairs mucociliary clearance (MCC), which is crucial for clearing pathogens from the respiratory tract. Cold and dry air can damage the epithelial lining of the respiratory tract, impairing immune defenses and increasing mucin production. Environmental factors also affect the host's antiviral immune responses. Low humidity can impair the production of interferon (IFN) and other antiviral genes, reducing the body's ability to fight viral infections. Temperature and humidity also influence the effectiveness of adaptive immunity, with high temperatures potentially weakening virus-specific immune responses. Conversely, maintaining optimal humidity levels can enhance immune defenses and reduce the risk of respiratory infections. The seasonal nature of respiratory viral infections is influenced by both environmental and behavioral factors. Indoor environments, particularly in winter, can have different humidity and temperature levels compared to outdoor environments, affecting virus transmission. Studies suggest that maintaining optimal indoor humidity levels can reduce the spread of respiratory viruses. Additionally, lifestyle factors such as handwashing and mask-wearing can help prevent the transmission of respiratory infections. In conclusion, environmental factors such as temperature and humidity play a critical role in the seasonal transmission and severity of respiratory viral infections. Understanding these factors can help in developing strategies to prevent and control respiratory infections, particularly during the winter months.The seasonal cycle of respiratory viral diseases has been recognized for thousands of years, with annual epidemics of the common cold and influenza occurring in winter in temperate regions. Epidemics caused by viruses such as SARS-CoV and SARS-CoV-2 also occur in winter. The mechanisms behind the seasonal nature of respiratory viral infections involve environmental factors and human behavior. Studies show that temperature and humidity affect the stability and transmission of respiratory viruses. Recent research highlights the importance of environmental factors in modulating host immune responses. This review discusses how outdoor and indoor climates influence transmission and host airway responses to viruses, and how these changes affect the seasonal circulation of respiratory viruses. Environmental factors such as temperature and humidity significantly influence the transmission of respiratory viruses. Low absolute humidity (AH) in winter leads to low indoor relative humidity (RH), which can enhance the survival and transmission of viruses. Studies show that respiratory viruses are more stable and transmissible under low RH conditions. Additionally, low humidity impairs mucociliary clearance (MCC), which is crucial for clearing pathogens from the respiratory tract. Cold and dry air can damage the epithelial lining of the respiratory tract, impairing immune defenses and increasing mucin production. Environmental factors also affect the host's antiviral immune responses. Low humidity can impair the production of interferon (IFN) and other antiviral genes, reducing the body's ability to fight viral infections. Temperature and humidity also influence the effectiveness of adaptive immunity, with high temperatures potentially weakening virus-specific immune responses. Conversely, maintaining optimal humidity levels can enhance immune defenses and reduce the risk of respiratory infections. The seasonal nature of respiratory viral infections is influenced by both environmental and behavioral factors. Indoor environments, particularly in winter, can have different humidity and temperature levels compared to outdoor environments, affecting virus transmission. Studies suggest that maintaining optimal indoor humidity levels can reduce the spread of respiratory viruses. Additionally, lifestyle factors such as handwashing and mask-wearing can help prevent the transmission of respiratory infections. In conclusion, environmental factors such as temperature and humidity play a critical role in the seasonal transmission and severity of respiratory viral infections. Understanding these factors can help in developing strategies to prevent and control respiratory infections, particularly during the winter months.