Buzz
The current flu season is proving to be particularly severe: As reported by the Washington Post, the hospitalization rate for flu is three times higher than for COVID-19 as of January 2025. But what causes the flu to have a designated season?
Flu viruses spread across the globe.
In temperate regions, flu outbreaks are more intense in the fall and winter months. The previously accepted notion was that influenza viruses either went dormant or existed at minimal levels during summer, only to resurface again. However, scientists have since discovered that, instead of just lying low in the off season, these viruses also travel across the globe and spread among populations worldwide.
In 2007, a team of researchers from Pennsylvania State University and the National Institutes of Health discovered that the influenza A virus uses its summer journey to interact with exotic viruses in tropical regions (where flu activity persists year-round), exchange genetic material, and then return with enough genetic variation to bypass our immune defenses. Scientists are still working to pinpoint the exact trigger that causes these reintroduced viruses to infect people during the fall and winter months.
Weather and climate play a significant role in the spread of influenza.
Influenza viruses thrive in cold winter temperatures and the accompanying dry air. They endure better in dry air compared to moist air and can survive longer on exposed surfaces like countertops, doorknobs, and keyboards when it's chilly. For humans, dry air can lead to dehydrated mucus and drier nostrils and airways, potentially making it easier for viruses to settle in once passed to us. A study on guinea pigs conducted at Mount Sinai School of Medicine revealed that flu transmission is more effective in cold (41°F) and dry (20 percent humidity) conditions, and transmission decreased as temperature and humidity increased (at 86°F and 80 percent humidity, transmission didn't occur).
Human behavior plays a major role in the spread of the virus.
The fall and winter months bring new school terms and colder weather, causing more people to gather indoors in close proximity, providing the perfect environment for viruses to spread from person to person. In tropical regions without distinct winters, where flu continues year-round, flu cases also tend to peak during the rainy season when people spend more time indoors together.
Our physiology makes us more vulnerable.
Due to spending more time indoors and shorter daylight hours in winter, our Vitamin D levels drop, which, along with other seasonal changes to our immune systems, could make us more vulnerable to the flu for a few months each year, acting as a “seasonal stimulus” for infection.
