Influenza

Seeing as the Netherlands is suffering some mild epidemic (over 51 out of every 100.000 people with flu-like symptoms), I might as well dedicate a post on the well-known and fearsome Influenza virus.

Influenza is a pesky disease affecting the respiratory system. I think about everyone's had it at some point in their lives.

Bacterial/Viral/Fungal/Parasitic: Viral
Causative agent: Influenza A / Influenza B / Influenza C
Incubation time: about 2 days
Antibiotics of choice: - 
Antivirals of choice: Adamantanes, amantadine, rimantadine, oseltamivir (Tamiflu), zanamivir (Relenza)
Toxins / Factors: - 
Bioterrorism agent category: ? (I think it's a potentially good weapon, though..)
Vaccine available: Yes, although none of them grant full immunity [1]

The info regarding Influenza has been updated as of January 11th, 2013.

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About the Virus
Flu is caused by several types of Influenza viruses, typed A or B. It is a RNA virus belonging to the orthomyxovirus group.
There are three types of flu viruses: A, B, and C. Type A and B most often cause disease in humans, which is why only type A and B viruses are included in the Dutch vaccination programme.
There are many different variations of flu viruses, as the virus has a tendency to mutate every once in a while. Therefore, getting the flu last year most often won't protect you for the coming year.

The virus itself carries 8 packages of RNA. As it is a virus, it cannot replicate itself, and it relies on host cells to do this. The 8 packages of RNA are 'injected' into a healthy cell for it to start reproducing the virus.
The influenza virus infects the respiratory epithelial cells.[1]

Symptoms
- Fever (sudden onset)
- Chills
- Fatigue
- Headache
- Cough
- General malaise (Feeling like crap)
- Vomiting and/or diarrhea (especially in children)
- Sore throat (uncommon)
- Nasal discharge (uncommon)

In bad cases, a simple flu can progress into pneumonia, as the virus has already weakened your body, making it susceptible to bacteria causing pneumonia.[1]

Risk groups
- Children <2yo
- Pregnant ladies and their fetuses
- Elderly people
- People with respiratory disease (asthma, bronchitis, etc)
- People with cardiac disease
- People with renal disease (renal = kidney)
- People with chronic underlying conditions (diabetes, etc)
- People with compromised immune systems
- People in nursing homes

These people are strongly urged to get a yearly flu shot. The Dutch government pays most, if not all, of the costs of the flu vaccination (that is, if you belong to the risk groups) [2][3]

Prevention
As the influenza virus is an airborne virus, staying inside all day might help.
Coughing in the elbow, washing your hands and staying away from other people if you yourself are infected with the virus will prevent it from spreading to others. 
The flu shot, while recommendable, does not offer full protection against the flu. This is because the scientists can only predict which strains of flu will visit us every year. Because there are so many strains, it's easy for one of them to sneak past them.[1]

Treatment
The antivirals I've listed above may be taken to shorten the duration of the illness. Healthy people do not need to visit the doctor or the hospital, and should stay at home and wait for their immune systems to kick the virus out.
Those who are at risk may need to be hospitalized.[2]  

Okay, seems nice and all, but why do we keep bitching about it?
Because there are so many varieties of the virus, it is very possible for multiple viruses to infect a creature at the same time. The moment this occurs, and replication of the virus is started inside the host cells, these viruses might combine into a whole new strain.
Some strains of influenza only affect birds, whereas other strains affect mammals. Swine and pigs can carry both avian and human flu.
When a deathly strain of avian flu infects a pig, along with a mild strain affecting humans, the virus might combine into a deathly strain capable of wiping out thousands of humans.
Before the scientists have managed to create a vaccine, thousands of humans might have already passed away, as this new virus might be ridiculously infectious.[1]

Notable pandemics
- Russian Flu, year 1889, Strain: H2N2
- Old Hong Kong, year 1900, Strain: H3N8
- Spanish Flu, year 1918, Strain: H1N1
- Asian Flu, year 1957, Strain: H2N2
- Hong Kong Flu, year 1968, Strain: H3N2
- Swine Flu, year 2009, Strain: H1N1 [1]

[1] Madigan, M., Martinko, J., Stahl, D., Clark, D., Brock Biology of Microorganisms, 2011, Pearson
[2] WHO, Influenza (Seasonal), April 2009,
http://www.who.int/mediacentre/factsheets/fs211/en/index.html
[3] RIVM, Voor wie is de jaarlijkse griepprik?, date unknown,
http://www.rivm.nl/Onderwerpen/Onderwerpen/G/Griepprik/Voor_wie_is_de_jaarlijkse_griepprik