influenza

Any of various viral infections primarily affecting the air passages, accompanied by systemic effects such as fever, chills, headache, joint and muscle pains, and lassitude. Treatment is with bed rest and analgesic drugs such as aspirin or paracetamol.

Depending on the virus strain, influenza varies in virulence and duration, and there is always the risk of secondary (bacterial) infection of the lungs (pneumonia). Vaccines are effective against known strains but will not give protection against newly evolving viruses. The 1918–19 influenza pandemic (see epidemic) killed about 20 million people worldwide.

In the USA flu affects an average 10–20% of the population annually with 20,000 deaths from flu-related complications.

In 2001, at least part of the reason for the virulence of the 1918 influenza pandemic was discovered. A research team at the Australian National University analysed genetic sequences of tissue samples taken from people who died in the pandemic and whose bodies had been preserved in permafrost. The scientists found that a particular gene in the 1918 virus was a mixture of previously existing pig and human influenza viruses. Analysis showed that this gene splicing, which would have made the virus unrecognizable to the human immune system, occurred just before the 1918 outbreak.