What is influenza?

According to the World Health Organisation (WHO), influenza infects between 5-15% of the world population and causes severe illness in up to 5 million people each year. It is responsible for between 250,000 and 500,000 deaths per year, worldwide.

Overview

Influenza (more commonly known as ‘flu’) is a highly contagious infection of the lungs and upper airways that is caused by catching one of the three flu viruses, influenza A, B or C. The virus spreads in small droplets of saliva which are coughed or sneezed into the air by infected people.

After a short incubation period (1-4 days), the symptoms begin to appear, often quite suddenly. People are usually infectious for about a day before symptoms are shown and remain infectious for 5 days after the onset of symptoms. Children and the elderly may be infectious for longer periods if they have a lowered immune system. Infectious people should try and avoid contact with others until they feel better.

Influenza epidemics usually occur during winter months (October – April in the northern hemisphere) due to cold, damp conditions that allow it to survive longer outside the body. In equatorial climates, this seasonal effect is less pronounced.

Symptoms

Symptoms usually peak after 2-3 days and last for 5-8 days Flu symptoms can be confused with having a heavy cold. Both flu and the common cold are caused by viruses but flu has a greater impact on the lungs and is associated with a more severe general illness. There may be less nasal congestion than occurs with a cold.  

Main symptoms:

• High temperature (fever) that develops quickly (38˚C/ 100.4˚F or above)
• Sweating
• General aches and pains all over body
• Dry, chesty cough
• Sore throat
• Sneezing
• Loss of appetite
• Difficulty sleeping
• Vomiting and diarrhoea (particularly in children).

Although the fever, sore throat and general illness get better after a week, symptoms such as cough and tiredness may last up to 3 weeks.

How does influenza change?


(image courtesy of istockphoto/ Sebastian Kaulitzki)
The ‘flu virus changes it’s characteristics frequently. Usually these changes are small, and the new virus is similar to previous strains. Although these new strains cause epidemics every few years, many people will have some immunity so only a minority of people will be susceptible and catch ‘flu
 
Occasionally the ‘flu virus undergoes a big change resulting in a completely new strain which causes a pandemic of ‘flu.  Pandemic flu occurs every few decades and rapidly spreads across most countries because populations have no immunity to new strains of the virus. Symptoms are the same as normal flu but may be more severe.

Increased use of air travel makes it easier for viruses to spread around the world at a faster rate than they would have done during previous outbreaks.

Learn about pandemic flu.

Future implications

The World Health Organisation (WHO) predicts that the next flu pandemic could kill up to 650,000 people. Developing countries are likely to be most at risk because they have limited healthcare and people may already have illnesses as a result of poor diets.

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