Arguably one of the most prevalent of the virus families, the orthomyxo virus family is composed of three strains of influenza, A, B and C. In the outbreak of 1918, over 20 million people died in 4 months, more than the war itself or the Plague. Additionally, these viruses strike annually, infecting millions of people with new strains of the virus and resulting in characteristic reports of "flu-like" symptoms.

- ssRNA (-) sense genome
- Linear genome with 7-8 segments
- Replication occurs in the nucleus
- Steals caps from nucleus for viral mRNA
- Envelope lines with hemagglutinin (H) and neuraminidase (N) peplomers

Epidemiology and clinical representation
- transmitted through respiration and appears in the winter months
- the strains are generally host specific
- birds serve as the host sink and carry the various strains without symptoms
- pigs serve as a medium where bird and human strains are able to cross and produce new strains

Click here to watch how new strains are formed! (Nat'l Foudation for Infectious Diseases)

Symptoms begin abruptly and include:
- high fever
- cough and respiratory discharge
- muscle pains
- chills
- normal white blood cell count
- loss of appetite
- headache

Shift versus Drift

The virus is able to reappear and infect populations annually because it annually changes the combinations of hemagglutinin and neuraminidase subgroups that are present on its surface. These changes can occur in two fashions and will dictate the impact of the viral strain. Antigenic drift refers to a mutation that results in a slight change in the genome and hence the impact of the virus is relatively minimal. However, occasionally, antigenic shift occurs when the reading frame becomes shifted. This results in a dramatic change in the combination of surface proteins and has historically resulted in pandemics. Influenza type A is able to undergo both types of change, whereas type B can only undergo genetic drift. Current strains of type A are H1N1 and H3N2, the former being the same combination as caused the pandemic of 1918. Some examples of pandemics resulting from shift events are:

- 1918-9: The "Spanish Flu" (H1N1) killed 500,000 people in the US and 20 million worldwide
- 1957-8 : The "Asian Flu" (H2N2) killed 70,000 people in the US
- 1968-9: The "Hong-Kong Flu" (H3N2) killed 34,000 people in the US

Because of the potentially devastating impact that the virus can have, means have been developed to minimize its threat.

Vaccines and Antiviral Agents
Annually, by examining strains that appear at the end of the viral season, researchers make guesses about the strains of influenza that will emerge the following year. According to these estimates, a trivalent inactivated vaccine is developed (containing two strains of type A and one strain of type B). Because their estimation was correct last year, this year’s vaccine was for the correct subtypes and resulted in a relatively mild flu season:

1999-2000 Vaccination
Northern Hemisphere
A/Sydney/5/97 (H3N2)
A/Beijing/262/95 (H1N1)
Southern Hemisphere
A/Caledonia/20/99 (H1N1)
A/Moscow/10/99 (H3N2)

Additionally, there exist several types of antiviral drugs that either target the uncoating process in type A virus or result in neuraminidase inhibition:

Target uncoating: amantadine (Symmetrel) and rimantadine (Flumadine)
Neuraminidase inhibitor: oseltamivir (Tamiflu) and zanamivir (Relenza)