The Flu Bug, does it Go On Vacation in The Summer?

For the past 35 years, flu activity in the US has peaked in February, according to the Centers for Disease Control and Prevention, so the shortest month of the year might not feel that way for the many infected. But February isn’t the only month you can contract the flu. In fact, it’s always flu season somewhere in the world. In the Northern Hemisphere, including the US, “flu season typically picks up around October, and it can usually peak between December and February,” says Amanda Simanek, associate professor of epidemiology at the University of Wisconsin-Milwaukee’s Joseph J. Zilber School of Public Health. “But we can see cases as late as into April and May,” she adds. In the Southern Hemisphere, it’s the opposite. Flu season typically runs from April to September and often peaks during August, although it varies by region. These are the winter months for the Southern Hemisphere, which brings us to a common theme: Influenza and winter go together like hot chocolate and marshmallows.

Why flu season is in the fall and winter? The flu tends to spike in the fall and winter for a major reason: the temperature. “The virus survives better in cool, dry temperatures,” Simanek said. And that’s thanks to a protective gel-like coating that surrounds the flu virus while it’s in the air. The flu is an airborne infectious disease. So in order to spread, the virus needs to survive long enough in the air to travel from one person to the next. And that’s where the virus’s gel-like coating helps when it’s cold outside. In colder temperatures, that capsule, which is made of fats and oils called lipids, hardens into a shell around the virus. This protects the virus and keeps it alive long enough to spread between victims. For comparison, this isn’t as easy to do when it’s warm outside because the lipid coating degrades, exposing the virus to the environment where it can easily get destroyed before finding a host. Once the virus is inside you, your body temperature degrades the coating, releasing the virus. After that, the virus starts to wreak havoc and replicate in hopes of infecting you and the next bystander. If you get the flu, you will be contagious for 5 to 7 days after your symptoms begin. It doesn’t help that people tend to congregate inside in close quarters when it’s cold and dreary outside, making it that much easier to spread those tiny respiratory droplets containing the virus.

The best way to prevent the flu is to get a flu shot as soon as it is available, which is usually in October. Yes, you can get the flu when it’s not flu season. Once the weather warms up and the spring flowers start to bloom, flu activity tends to drop off. But that doesn’t mean you can’t still get the flu. “When you come into contact with a flu virus or respiratory virus to which you are susceptible, you can certainly catch that, no matter what season it is,” says Lisa Maragakis, MD, senior director of infection prevention for the Johns Hopkins Health System. Plus, when it’s summer in North America, it’s winter in the Southern Hemisphere. So, for example, if you travel to Australia during their winter, you could be exposed to the flu there.

I recently came across an article that says the flu virus migrates from the northern hemisphere to the southern hemisphere. I have shown that this is erroneous. Bu it does mostly become more dormant or less prevalent. But during this time it continues to mutate in it’s hosts, so when it becomes cooler and it has a more friendly environment to spread in, it is basically a new strain or strains, that the general population has little or no immunity to. Some years the mutations are fewer and the virus becomes less of an issue. Is this going to happen to the cornavirus? Well it doesn’t seem to have a season. It is happy all year long. Lucky us.

Resources:, “When is flu season and why there is a flu season in the first place,” Jennifer Larson;, “The Reason for the Season: why flu strikes in winter,” By Hannah Foster PHD;


The Reason for the Season: why flu strikes in winter:

What is the Flu?

In order to discuss why we have a flu season, we must first understand what the flu is. The flu, also called influenza, is a viral respiratory illness. A virus is a microscopic infectious agent that invades the cells of your body and makes you sick. The flu is often confused with another virus, the common cold, because of the similarity in symptoms, which can include a cough, sore throat, and stuffy nose. However, flu symptoms also include fever, cold sweats, aches throughout the body, headache, exhaustion, and even some gastro-intestinal symptoms, such as vomiting and diarrhea.

The flu is highly contagious. Adults are able to spread the virus one day prior to the appearance of symptoms and up to seven days after symptoms begin. Influenza is typically spread via the coughs and sneezes of an infected person. Around 200,000 people in the United States are hospitalized each year because of the flu, and of these people, about 36,000 die.  The flu is most serious for the elderly, the very young, or people who have a weakened immune system.

The Flu Season

The flu season in the U.S. can begin as early as October, but usually does not get into full swing until December. The season generally reaches its peak in February and ends in March. In the southern hemisphere, however, where winter comes during our summer months, the flu season falls between June and September. In other words, wherever there is winter, there is flu. In fact, even its name, “influenza” may be a reference to its original Italian name, influenza di freddo, meaning “influence of the cold”.

A common misconception is that the flu is caused by cold temperatures. However, the influenza virus is necessary to have the flu, so cold temperatures can only be a contributing factor. In fact, some people have argued that it is not cold temperatures that make the flu more common in the winter. Rather, they attest that the lack of sunlight or the different lifestyles people lead in winter months are the primary contributing factors. Here are the most popular theories about why the flu strikes in winter:

1) During the winter, people spend more time indoors with the windows sealed, so they are more likely to breathe the same air as someone who has the flu and thus contract the virus.

2) Days are shorter during the winter, and lack of sunlight leads to  low levels of vitamin D and melatonin, both of which require sunlight for their generation. This compromises our immune systems, which in turn decreases ability to fight the virus.

3) The influenza virus may survive better in colder, drier climates, and therefore be able to infect more people.

The Flu Likes Cold, Dry Weather

For many years, it was impossible to test these hypotheses, since most lab animals do not catch the flu like humans do, and using humans as test subjects for this sort of thing is generally frowned upon. Around 2007, however, a researcher named Dr. Peter Palese found a peculiar comment in an old paper published after the 1918 flu pandemic: the author of the 1919 paper stated that upon the arrival of the flu virus to Camp Cody in New Mexico, the guinea pigs in the lab began to get sick and die. Palese tried infecting a few guinea pigs with influenza, and sure enough, the guinea pigs got sick. Importantly, not only did the guinea pigs exhibit flu symptoms when they were inoculated by Palese, but the virus was transmitted from one guinea pig to another.

Now that Palese had a model organism, he was able to begin experiments to get to the bottom of the flu season. He decided to first test whether or not the flu is transmitted better in a cold, dry climate than a warm, humid one. To test this, Palese infected batches of guinea pigs and placed them in cages adjacent to uninfected guinea pigs to allow the virus to spread from one cage to the other. The pairs of guinea pig cages were kept at varying temperatures (41°F, 68°F, and 86°F) and humidity (20%-80%). Palese found that the virus was transmitted better at low temperatures and low humidity than at high temperatures and high humidity (see Figure 1).

Figure 1 ~ Experimental Setup. Guinea pigs were housed in adjacent cages. Guinea pigs in cage 1 were infected by Palese with influenza. Palese observed how many guinea pigs in cage 2 became infected from the guinea pigs in cage 1 at different temperatures and levels of humidity. B, C) Transmission rates were 100% at low humidity, regardless of temperature. At high humidity, transmission occurred only at the lower temperature. 

However, Palese’s initial experiment did not explain why the virus was transmitted best at cooler temperatures and low humidity. Palese tested the immune systems of the animals to find out if the immune system functions poorly at low temperatures and low humidity, but he found no difference in innate immunity among the guinea pigs. A paper from the 1960s may provide an alternate explanation. The study tested the survival time of different viruses (i.e. the amount of time the virus remains viable and capable of causing disease) at contrasting temperatures and levels of humidity. The results from the study suggest that influenza actually survives longer at low humidity and low temperatures. At 43°F with very low humidity, most of the virus was able to survive more than 23 hours, whereas at high humidity and a temperature of 90°F, survival was diminished at even one hour into incubation.

The data from these studies are supported by a third study that reports higher numbers of flu infections the month after a very dry period. In case you’re wondering, this is only the case in places that experience winter. In warmer climates, oddly enough, flu infection rates are correlated most closely with high humidity and lots of rain. Unfortunately, not much research has been done to explain these contradictory results, so it’s unclear why the flu behaves so differently in disparate environments. This emphasizes the need for continued influenza research. Therefore, we can conclude that, at least in regions that have a winter season, the influenza virus survives longer in cold, dry air, so it has a greater chance of infecting another person.

Although other factors probably contribute as well, the main reason we have a flu season may simply be that the influenza virus is happier in cold, dry weather and thus better able to invade our bodies. So, as the temperature and humidity keep dropping, your best bet for warding off this nasty bug is to get your flu shot ASAP, stay warm, and invest in a humidifier.


+coughing or sneezing
+body aches and pains
+runny or congested nose
+sore throat

+fever over 103°F (39.4°C)
+cough that includes yellow, green, or brown mucus
+shortness of breath
+pain in your chest, particularly when breathing in
+light headedness, dizziness, or passing out
+persistent vomiting

Common Cold:

The common cold is another respiratory infection caused by a variety of viruses. There’s a lot of overlap between the symptoms of the common cold and those of the flu, such as a runny nose or congestion, coughing or sneezing, and sore throat. However, unlike the flu, the symptoms of the common cold develop gradually and are most often less severe. There are other differences between a cold and flu as well.

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