The "chyu-you (derived from the famous Chinese book)" means the balanced state or person with harmonized character, which is appreciated by people for a long time in Japan. I hope that people and countries should keep this word in mind, and exercise control over themselve. The host and the pathogen co-evolve to adapt to each other. It is time for humans to harmonize the world and society. Science is an important arena which gives us many opportunities to achieve this. Effect modification of environmental factors on influenza-associated mortality: a time-series study in two Chinese cities BACKGROUND: Environmental factors have been associated with transmission and survival of influenza viruses but no studies have ever explored the role of environmental factors on severity of influenza infection. METHODS: We applied a Poisson regression model to the mortality data of two Chinese metropolitan cities located within the subtropical zone, to calculate the influenza associated excess mortality risks during the periods with different levels of temperature and humidity. RESULTS: The results showed that high absolute humidity (measured by vapor pressure) was significantly (p < 0.05) associated with increased risks of all-cause and cardiorespiratory deaths, but not with increased risks of pneumonia and influenza deaths. The association between absolute humidity and mortality risks was found consistent among the two cities. An increasing pattern of influenza associated mortality risks was also found across the strata of low to high relative humidity, but the results were less consistent for temperature. CONCLUSIONS: These findings highlight the need for people with chronic cardiovascular and respiratory diseases to take extra caution against influenza during hot and humid days in the subtropics and tropics. Influenza used to be considered as a "cold" disease as it usually returns every cold winter in temperate countries. However, recent studies have shown that influenza can be active throughout the year in the warm tropics and subtropics and the disease burden of influenza there can be as heavy as that in temperate climates [1] [2] [3] [4] . It has been proposed that influenza seasonality is driven by complicated interactions between antigenic drifts of virus strains, environmental factors, host susceptibility and behavior changes [5] [6] [7] . Among these potential factors, environmental factors including temperature and relative humidity have been most thoroughly explored by laboratory and observational studies [8, 9] . Recent studies raised a hypothesis that absolute humidity is one of drivers for influenza seasonality in temperate regions [10, 11] , but the mechanism behind various seasonal patterns of influenza outbreaks under different climates remains unclear.