Why is influenza spreading in Japan in the summer?

Influenza has made an unusually early appearance in Japan, with cases surpassing a critical threshold in late August, according to the Mainichi Shimbun. This marks about one month earlier than the previous year and the second-earliest since the infectious diseases law took effect in 1999.

The number of reported cases per medical institution reached 1.11 between August 17 and 23, increasing to 1.76 by August 24-30, and could continue to rise. Influenza is typically linked to winter, so why is this outbreak happening now in the summer?

Several factors may explain this early outbreak. The extreme heat this year could have contributed, as people spent more time indoors with air conditioning, which might increase the risk of transmission when windows are closed and ventilation is poor. Additionally, summer fatigue could have weakened resistance by reducing appetite and sleep.

It's summer in the Northern Hemisphere, where Japan is located, but in the Southern Hemisphere, where the seasons are opposite, it's winter—the flu season. Viruses could have been brought in by tourists from the Southern Hemisphere or by Japanese travelers returning home from that region. Summer is a period of increased global travel, which can facilitate the spread of infections.

Influenza often brings about body-wide symptoms, such as a high fever, joint and muscle pain, and fatigue. In contrast, a common cold usually involves a sore throat, runny nose, and other symptoms, with less severe fever. To prevent the spread of flu, basic measures like frequent hand-washing, gargling, wearing masks in crowds, and ensuring good ventilation even while using air conditioning are recommended. Maintaining a balanced diet, adequate sleep, and overall good health is also crucial.