Have you ever wondered why the influenza virus is more potent and dangerous than the common cold virus? They’re both viruses, but what makes the influenza worse that it causes more deaths? Let's take a look at the science...
2017 has been a pretty bad year for flu in Australia. More than 160,000 people have been confirmed to have the flu this year, ranging from mild to severe, and some even causing death. So why is it the flu can cause death but we don’t generally hear of colds doing the same?
What is the cold virus?
Not all common cold viruses are created equally, and the course of a “cold” depends to a degree on which cold virus a person is afflicted by. These viruses share a tendency to infect and replicate most effectively in the cooler environment of the upper airway, including the ears, nose, throat and sinuses.
Rhinovirus is the most common virus responsible, and tends to cause an infection of the upper airway, lasting a few days and improves on its own. Typically, rhinovirus binds to a receptor on the surface of cells in the nose and sinuses. This allows the virus to enter cells, start to replicate, and causes infected cells to produce molecules that lead to inflammation. These inflammatory molecules are thought to be responsible for the symptoms associated with the common cold; runny or blocked nose, sore throat and cough.
But infection can be more severe in patients with an impaired immune system, including those with cancer, on medications to suppress the immune response, or those undergoing a bone marrow transplant. Plus, the common cold may exacerbate or contribute to the development of chronic airway diseases, like asthma.
What is the flu?
The illness you get when you catch influenza (the flu) varies widely, and depends on the person infected (the host), the virus, and secondary bacterial infections that may follow infection with the virus. The flu virus can infect both the upper and lower respiratory tract, which in part explains its ability to cause severe disease.
Other medical conditions, age and a weakened immune system also play a big part in how much the flu virus will affect the host. Plus, a person’s genetic makeup may predispose them to a more severe inflammatory response, known as a “cytokine storm”. Inflammation is a normal response that allows our body to fight infection and heal damaged tissue. Paradoxically, too much inflammation makes the illness from flu worse by causing more swelling and tissue destruction.
For the flu virus to bind to and infect cells in our respiratory tract it uses a molecule called “hemagglutinin”. Bird and swine flu strains, which cause pandemics in humans may have a haemagglutinin molecule on their surface that preferentially binds to receptors in the lower respiratory tract, including the tiny air sacs (alveoli). These alveoli are responsible for transferring oxygen into and taking carbon dioxide out of the blood stream.