Sick with the flu? Here’s why you feel so bad

June 14, 2019 6.16am AEST

You might feel terrible. But your runny nose, sore throat
and aches are signs your body is fighting the flu virus. And that’s a
good thing.
from www.shutterstock.com

Author

1. Stephen Turner

   Professor, viral immunology, Monash University

Disclosure statement

Stephen Turner receives funding from the National
Health and Medical Research Council of Australia, the Australian
Research Council and the National Institutes of Health Centre of
Excellence for Influenza Research and Surveillance.

Partners

“You never forget the flu”. This is the title of the Victorian health department’s current campaign, which highlights people’s recollections of having the flu.

Phrases include “I’ll never forget the pain of the fever”, “the flu flattened me”, “the flu knocked me out for weeks”.

This gives the impression that when you have the flu, you know you
have it. What makes the flu so memorable is the severe symptoms. These
include fever, aches and pains, a sore throat, runny nose, cough, and
feeling weak and lethargic.

But what causes the flu? And why are the symptoms so severe?

Read more:
Health Check: how long should you stay away when you have a cold or the flu?

What causes the flu?

Influenza is caused by a virus,
a small microbe that needs to enter our cells to replicate and produce
more viruses. The influenza virus infects cells that line our airways
and so is easily transmitted via the spread of droplets released when we sneeze or cough.

Coughs, sneezes and the other symptoms we feel after getting the flu, are largely due to our bodies fighting the infection.

Read more:
I’ve always wondered: why is the flu virus so much worse than the common cold virus?

The immune response is a double-edge sword

When you are infected with the flu virus, your innate immune system
kicks in. Special receptors recognise unique parts of the virus,
triggering an alarm system to alert our bodies that an infection is
under way.

This produces a rapid but non-specific response — inflammation.

Read more:
Explainer: what is the immune system?

Inflammation results from the action of small proteins called
cytokines. A primary role of cytokines is to act locally in the lung to
help limit the initial infection taking hold.

They can also make their way into the circulation, becoming systemic
(widespread in the body) and act as a “call to arms” by alerting the
rest of the immune system there is an infection.

Unfortunately, your body’s inflammatory response, while trying to
fight your infection, results in the flu symptoms we experience.

Inflammation can trigger increased mucus production. Mucus (or
phlegm) is a sticky substance that helps capture virus in the lungs and
upper airways. The increased amount of mucus in the airways can trigger
coughing and/or sneezing, and can lead to a runny nose. This helps expel
the virus from our body before it can infect other airway cells.

Read more:
Health Check: what you need to know about mucus and phlegm

Inflammation also results in an increase in body temperature or
fever, which creates an inhospitable environment for the flu virus to
replicate.

While an increased body temperature helps fight the infection, it
also results in you feeling colder than usual. That’s because you feel a
greater temperature difference between your body and the outside
environment.

This can induce rapid muscle contractions in an effort to heat you
up. This is why you can feel like you can’t stop shivering while at the
same time burning up.

Read more:
Monday’s medical myth: feed a cold, starve a fever

Finally, some of these inflammatory molecules act directly on
infected cells to stop the virus replicating. They can do this by either
interfering with the replication process directly, or alternatively, by
actually killing the infected cell.

One of these factors is tumour necrosis factor alpha
(TNF-alpha). While its actions limit where the flu virus can replicate,
its side effects include fever, loss of appetite and aching joints and
muscles.

Calling in the big guns

Inflammation induced by the innate response also helps alert the adaptive immune system that there is an infection.

While innate immunity provides an immediate, albeit non-specific,
response to viral infection, it is the adaptive immune response that can
efficiently clear the infection.

The adaptive immune system consists of specialised white blood cells
called T and B cells that when activated provide a highly specific
response to infection.

Activation of flu-specific T and B cells in tissues called lymph
nodes results in the generation of hundreds of thousands of clones, all
specific for the flu virus. These can migrate into the lungs and
specifically target the virus and its ability to replicate.

This enormous expansion of T and B cell numbers in response to
infection results in swelling of the lymph nodes, which you can feel
under your armpits or chin, and which can become sore.

Flu-specific T cells are also a source of the inflammatory molecule
TNF-alpha and help fight influenza infection by killing off
virus-infected cells. Both actions can contribute to the flu symptoms.

Why can flu become a serious problem?

Our ability to see off a flu infection requires a coordinated response from both our innate and adaptive immune responses.

If our immune system function is diminished for some reason, then it
can prolong infection, lead to more extensive damage to the lung and
extended symptoms. This can then result in secondary bacterial
infections, leading to pneumonia, hospitalisation and eventually death.

Then there are people whose immune system doesn’t work work so
efficiently who are particularly susceptible to the flu and its
complications. These include:

• the very young, whose immune system is still yet to mature
• the elderly, whose immune system function wanes with age
• people with other conditions where immune function might be
  compromised, or be taking medication that might suppress the immune
  system.

Preventing the flu

Washing your hands and covering your mouth when coughing and sneezing
are simple things we can all do to reduce the chance of catching the
flu in the first place.

And getting the flu vaccine activates your adaptive immune response
to induce the sort of immunity efficient at protecting us from
infection.

With the flu season well under way, prevention is our best bet that you won’t be saying “Remember the time I got the flu”.