When Is Flu Season and Why Is the Flu Seasonal?

When Is Flu Season and Why Is the Flu Seasonal?
Nobody likes getting sick, certainly not with the flu. As the common cold’s bigger, meaner cousin, the influenza virus can be particularly nasty. Although it is very difficult to determine exact numbers because the vast majority of flu cases go unreported, the Center for Disease Control (CDC) has done studies that suggest there are between 9 and 35 million instances of the flu each year in the United States alone. Included in this are the serious cases requiring hospitalization, of which there are 140,000 to 710,000 each year. Although flu-related deaths are extremely hard to pinpoint, as influenza-like symptoms are so often combined with other diseases and ailments, they estimate between 12,000 and 56,000 each year, 80 percent occurring among people who are unvaccinated.

When Is Flu Season?

So, clearly flu season is not to be taken lightly. Unfortunately, it is impossible to predict exactly how severe each flu season will be, as different strains and weather patterns lead to differing results. What we do know is that it really only occurs in cooler weather, being at its worst in fall and winter. In the United States, flu season generally starts at the end of October and runs until the end of March, typically peaking in February.

Why is this, though? Why is the flu seasonal? Well, there are a number of contributing factors to the seasonal nature of the flu.

Why Is the Flu Seasonal?

Weather and Climate

The influenza virus thrives in cold and/or dry air. These conditions allow it to survive longer, making it more likely to be passed from person to person. In winter, the virus will last much longer both in the air (following sneezing or coughing) and on surfaces (sneezing, coughing, and touching). Ideal conditions exist for the flu virus when the temperature is under 41 degrees Fahrenheit and humidity is under 20 percent.

Meanwhile, the flu basically disappears in summer because of increased warmth and humidity. At above 86 degrees and 80 percent humidity the virus does not transmit at all, making it essentially dormant. Although not entirely. Scientists have determined that flu viruses actually migrate around the world to places where conditions are more conducive to transmission, where they then mix with other viruses. This is why the flu is always slightly different each year, because the viruses return as new strains.

Close Contact

In summer, the weather is warmer, the days are longer, and people normally spend much more time outdoors. Throughout fall and winter people typically spend a much higher percentage of their time indoors, leading to closer contact and more transference of viruses. This is a trend that has even been noticed in warm regions where the flu is more prevalent when people are forced indoors during the rainy season.

The Sun

One of the keys to resisting the flu is vitamin D, which we receive mainly from sunlight. With shorter days and more time indoors in winter, we do not get as much vitamin D and become more susceptible to viruses.

More Facts About the Flu


Flu symptoms only start 1 to 4 days after the virus is contracted, which means a person can be contagious from 1 day before symptoms appear to one week after. The most dangerous period is during the first 3 to 4 days of symptoms, although it is actually possible to be infected and contagious but show no symptoms. The nature of influenza symptoms makes catching the flu pretty easy, since you can be exposed to the virus by someone who is seemingly healthy.


The influenza virus can travel up to 6 feet, which is why the most common method of spreading the disease is through coughing, sneezing, or talking. You contract it through infected liquids that enter your mouth, nose, or eyes. It can also be transferred if you touch an infected surface, such as a doorknob, railing, phone, or keyboard, and then touch your face.

Therefore, the key is to stay as far away from sick people as possible. When it’s not possible, wash your hands often, avoid touching your mouth, nose, and eyes, and do your best to stay out of the direct line of fire of a coughing or sneezing family member or colleague.

How to Prevent the Flu

The CDC recommends annual flu shots for everyone 6 months of age or older. It normally takes about 2 weeks after taking the vaccine for your body to generate enough antibodies to protect itself against the virus, so it is best to get vaccinated before the end of October. However, as with so many health-related precautions, it is still better late than never.

Flu season in the United States kicked off early this year. The first reported casualty of the 2018-19 season occurred in early October when an unvaccinated child in Florida died of the flu. Despite the early start, scientists are expecting a relatively mild flu season based on recent data from the southern hemisphere. Of course, there are too many contributing factors to predict flu patterns with any real certainty, so it is always best to take all reasonable precautions.