Flu symptoms

Flu season tends to start as early as October and extend through the Winter months in the Northern Hemisphere. The seasonal flu virus changes somewhat each year due to genetic mutation, and can cause a mild or severe season, based on the number of people who fall ill, miss work, and have complications. Sometimes a pandemic strain will infect outside of the normal seasonal cycle, just as H1N1 did in 2009.

Flu symptoms tend to associate around the fact that the virus infects the respiratory tract and the immune response to its presence: coughing, sore throat, fever, muscle aches, headache, fatigue, runny or stuffy nose. Pandemic strains tend to also have vomiting and diarrhea included in the symptoms.

Recognizing flu symptoms

What is influenza?

Types of Influenza Virus

The flu is caused by the influenza virus. Three types of influenza are considered:

A - regularly circulate in humans and animals, some strains are species-specific, strains actively share mutations causing yearly shifts in circulating strains, two expected strains are included in each year's vaccine.

B - generally only infects humans, more mild infection than type A but can occasionally cause outbreaks, mutate slowly via genetic drift, expected seasonal strain is included in each year's vaccine.

C - cause mild respiratory illness, not attributed to outbreaks, not included in vaccine.

Type A virus is further broken down into subtypes based on two things: hemagglutinin (H), a binding protein expressed on the virus surface; and neuraminidase (N), an enzyme the virus expresses that breaks down mucus to help it infect the respiratory tract. There are 16 different H types and 9 different N types that a virus can express.

These A subtypes are even further broken down based on species, date, and geography because of the tremendous amount of mutation these viruses can undergo in a short period of time.