Causes of flu

What causes flu and how does it spread?

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Dr. Jen Tan
Immune System Expert
@AVogelUK
Ask Dr. Jen Tan

Flu is caused by viruses

Flu, or influenza, is caused by the influenza viruses.

A virus is an infectious agent which cannot multiply without inhabiting other cells. It is very small - 10,000 times smaller than the width of a human hair. Human cells act as hosts for viruses.  When a cell is infected, the virus takes control and uses the resources of the cell to produce more viruses.

It is because the virus cell inhabits a healthy cell that infections cannot be easily treated as this would also kill healthy cells. Antibiotics are effective for bacterial infections but have no impact on viruses.

The flu virus has many strains. Each can mutate regularly, making it even more difficult to prevent and treat the infections. This is why people can catch the flu over and over again.

How do flu viruses spread?

The most common way for viruses causing colds and flu to spread is from ‘hand to nose’.

Touching a surface infected with viruses (door handles, escalator rails or shaking hands with someone with the flu) passes it to your mouth, nose or eyes. Just observing how often people touch their faces will give you a clue as to how easily viruses can spread in this way.

Another way in which infective viruses are spread is in droplets produced when sneezing or coughing. It may surprise you that this is not as efficient in spreading infection as the ‘hand to nose’ route.

How do these viruses cause symptoms?

The immune system exists to protect our body against infection. It is made up of special blood cells, proteins, organs and other tissues - all designed to defend and support our body against invading organisms.

Put another way, it is the army protecting the body against infection by viruses, bacteria and fungi, keeping us healthy and free from infections.

When a virus invades the human body, it attaches to the cells lining the respiratory system. Upon invading the cell, they take over the host cell in order to thrive and multiply.

The immune system recognises this invasion and fights it by sending out other chemicals to counteract the multiplication of the flu virus. These chemicals cause inflammation of the mucous lining, resulting in a fever, sore throat, coughing, blocked nose and sinuses. It is the immune system fighting off the infection, which causes the symptoms rather than the flu virus itself.

A short history of flu pandemics

Flu was first described by Hippocrates in 412 BC. The first flu pandemic was first recorded in about 1580 and since then, there have been recurrences approximately every 20 years. A pandemic is the spread of a new disease throughout the world. It comes from the Greek word pandemos  meaning ‘pertaining to all people.’

In 1918-1919 the worst flu pandemic to date occurred. It became known as the Spanish flu although it affected up to 40% of the world’s population killing more than 5%. It was an unusual strain of the flu which seemed to target the young and healthy, and mortality rates were highest among those between 20 and 50 years of age.

2007 saw the start of a potential flu pandemic, although preventative measures stopped it from spreading out of control. This was known as bird flu as the strain of the virus adapted to infect birds. It is easily spread from avian to human, although human to human infection is more difficult without close and lasting contact. Bird flu pandemic scares have been recurring ever since.

The most recent flu pandemic occurred in 2009-2010. It became known as swine flu because the virus was a mutant of the Eurasian pig flu virus. It was not spread by eating pork products but by droplets in the air, as with normal flu.

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