Flu season is the portion of the year in which there is a regular outbreak in flu cases. It occurs during the cold half of the year in each hemisphere. Flu activity can sometimes be predicted and even tracked geographically. While the beginning of major flu activity in each season varies by location, in any specific location these minor epidemics usually take about 3 weeks to peak and another 3 weeks to significantly diminish. Individual cases of the flu however, usually only last a few days. In some countries such as Japan and China, infected persons sometimes wear a surgical mask out of respect for others.