Flu season

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Flu season is mostly a colloquial term used to describe the regular outbreak in flu cases, or even cases of the common cold during the late fall or winter. Viral activity such as the flu can sometimes be predicted and even tracked geographically. Flu season is most apparent in schools because the closer interactions and lower hygiene cause easier transmission. Some workplaces offer paid sick days, partly to prevent sick individuals from entering the workplace biome. Most often these minor epidemics take 3 weeks to peak, and another 3 weeks to significantly diminish.^{[#endnote_time]} Individual cases of the cold or flu however, usually only last a few days. In some countries such as Japan and China, infected persons sometimes wear a surgical mask out of respect for others.

More recently, flu vaccinations have made efforts to stop flu season pre-emptively. Since the Northern and Southern Hemisphere have winter at different times of the year, there are actually two flu seasons each year. Therefore, the World Health Organization (assisted by the National Influenza Centers) makes two vaccine formulations every year; one for the Northern, and one for the Southern Hemisphere.

While most influenza outbreaks in the Northern Hemisphere tend to peak in January or February, not all do. For example, the influenza pandemic of 1918 and 1919 reached peak virulence during late spring and summer worldwide, and not until October in the US. It remains unclear why outbreaks of the flu occur seasonally rather than uniformly throughout the year.

One possible explanation is that, because people are indoors more often during the winter, they are in close contact more often, and this promotes transmission from person to person. Another is that cold temperatures lead to drier air, which may dehydrate mucus, preventing the body from effectively expelling virus particles. The virus may also linger longer on exposed surfaces (doorknobs, countertops, etc.) in colder temperatures. Increased travel and visitation due to the holiday season may also play a role. ^{[#endnote_two]}

The garden variety flu that comes around every year is caused by flu virus species Influenzavirus A, Influenzavirus B, or Influenzavirus C. The species A varieties that yearly attack humans are called "human flu virus" which is to say it is a variety of the species "avian flu virus" (species A) that has made genetic changes to adapt to its human hosts. It passes from human to human all year round and never goes away completely. When it is cold (winter in the north, summer in the south part of the world) infection from "human flu" increases something like ten fold or more. Different strains of avian flu virus circulate in different years as it is constantly mutating. The flu vaccine for the 2005 - 2006 flu season contains proteins from the coat of two subtypes of species A and from species B. Species B and C don't have subtypes.

The annual flu (also called "seasonal flu" or "human flu") in the U.S. "results in approximately 36,000 deaths and more than 200,000 hospitalizations each year. In addition to this human toll, influenza is annually responsible for a total cost of over $10 billion in the U.S." [link].

The annually updated trivalent flu vaccine consists of hemagglutinin (HA) surface glycoprotein components from influenza H3N2, H1N1, and B influenza viruses. [link] The dominant strain in January 2006 is H3N2. Measured resistance to the standard antiviral drugs amantadine and rimantadine in H3N2 has increased from 1% in 1994 to 12% in 2003 to 91% in 2005. [link] [link]

"[C]ontemporary human H3N2 influenza viruses are now endemic in pigs in southern China and can reassort with avian H5N1 viruses in this intermediate host." [link]

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