Flu Influenza Season

In general, the cold winter months are when flu-causing viruses thrive and the warm summer months are when bacterial infections peak.

The flu season is typically defined as beginning in early winter to ending in early spring; more specifically, it lasts from November to mid-March of the following year. Flu is caused by human influenza viruses, typically influenza type A and B. These viruses are enveloped single-stranded RNA viruses that can replicate very quickly, depending on the host environment and other factors.

Viruses are not characterized as living organisms. They can remain dormant for a long period of time until an appropriate host is encountered. Once the cells of the host are infected, the virus is activated and begins to self-replicate. The ability of the virus to stay inactive is key to survival in suboptimal conditions, such as during the summer months.

Viral Adaptation

Scientists have proposed several different hypotheses that outline why viruses tend to thrive during the colder winter months.

One recent study found that viruses actually adapt to the environment they are exposed to. In cold temperatures, some viruses can create a hard coating around them, acting as a shell so that it can spread throughout the air more effectively and without being disrupted. Once the virus enters a host, the warm environment sheds the coating and the virus begins to infect cells and reproduce.

Another study points out that humidity plays a role in viral survival and transmission. Influenza viruses spread via small respiratory droplets released during a cough or sneeze. During the colder months, the air is much drier, and therefore, viruses within respiratory droplets can be transmitted more efficiently. The increased humidity observed during the summer months reduces the spread of the viruses. Animal studies show that the virus is most infectious when the humidity is around 20 percent, and not infectious at all when the humidity reaches 80 percent. In addition, the virus appears to be shed two days longer at 41 degrees Fahrenheit than at 68 degrees Fahrenheit.

Other Reasons for Why We Have a Flu Season

Perhaps we see increases in viral infection during the winter because people congregate together more frequently in confined spaces during the winter. Family and holiday gatherings provide ideal grounds for human-to-human contact and subsequent transmission of influenza viruses.

Each year, the Centers for Disease Control give a flu season outlook. This flu season, officials are especially concerned about the severity and extent of the H1N1 virus (“swine flu”). Flu shots are highly recommended for high-risk individuals. Initial reports indicate the outbreaks are not as severe or extensive as originally thought. However, the winter season has not reached its coldest temperatures yet, so a resurgence of H1N1 cases may be on the horizon.


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