What is Flu?
Flu is a contagious and infectious respiratory disease caused by the influenza virus which mainly affects the nose, throat, bronchi and, in certain cases, the lungs. Flu is different from the common cold which is caused by other respiratory viruses that typically develop quickly and with symptoms such as high fever, aching muscles, headache, a blocked nose (Rhinosinusitis) and, in more severe cases, difficulty breathing.
Flu is a seasonal disease and generally emerges in winter; in the Northern Hemisphere, it predominantly circulates from November to March. However, some years the virus may be active from October until the end of April.
Most people recover from influenza on their own. However, in young children, pregnant women, elderly individuals and people with an immunosuppressed state or a chronic illness such as diabetes or asthma, flu can be more serious and even fatal if it is not diagnosed and treated early.
There are three types of influenza virus: A, B and C. Types A and B are the most important because they lead to epidemics.
Type A is the most common and is responsible for causing the most severe epidemics, whereas type B causes more focalised outbreaks.
The type C influenza virus is detected less frequently and does not trigger epidemics. It only causes mild infections with very few symptoms.
A very important characteristic of type A influenza is its highly variable nature. The surface of the virus features two proteins or surface antigens, haemagglutinin (H) and neuraminidase (N). These two proteins are extremely important for both the virus, because they give the virus its capacity to infect its host, and humans, because the body produces defensive antibodies when it comes into contact with these proteins.
How many people does it affect?
According to estimates by the World Health Organization (WHO), some 10–20% of the world’s population are infected by the influenza virus every year. Based on these data, influenza is believed to cause 250,000–500,000 deaths due to respiratory infections resulting directly from the flu virus.
Those at the greatest risk of becoming infected and suffering complications due to influenza virus are pregnant women, children under 5 years, people over 65 years, patients with chronic conditions such as asthma, heart disease, lung disease or diabetes, immunosuppressed patients and healthcare personnel.
How does the influenza virus change each year?
Every year, type A influenza is subject to variations in its H and N proteins, consequently new subtypes of influenza A develop each winter. Therefore, at-risk groups need to get vaccinated every year because a new subtype of influenza could be circulating, and the latest vaccination is required to protect yourself from this winter’s subtype.
The influenza virus can change in two different ways:
Antigenic drift or minor antigenic changes. Antigenic drift is when small, gradual variations occur in the H and N protein antibody binding sites. These changes can make the virus resistant to antibodies developed to protect the individual in previous years and they take place in both type A and B influenza. This is the main reason why people get flu more than once during their lifetimes, because the antibodies generated in previous flu infections no longer provide fully effective protection against the new subtype of virus produced by the small changes. As such, flu vaccines have to be adapted every year to protect the population against the new influenza subtypes in circulation each flu season.
Antigenic shift or major antigenic changes. Antigenic shift is an abrupt, major change due to genetic exchange between human and animal influenza viruses that produces a new subtype which has never been in circulation before. This can result in outbreaks and global pandemics.
How is flu transmitted?
The influenza virus transmission routes are:
Human-to-human. By means of fine droplets expelled when someone with flu coughs, sneezes or talks. These droplets can reach the mouth, nose or eyes of anyone close by and infect them.
Hand contact with surfaces contaminated with droplets where an infected person has coughed or sneezed. Transmission occurs when the person touches their eyes, nose or mouth with their contaminated hands causing indirect infection.
So, people are most likely to get infected in busy places such as childcare centres, schools, retirement homes, hospitals or those with surfaces that are constantly touched by many hands such as the underground or buses. The most effective means of prevention in these circumstances is washing your hands regularly. And if you have flu, please use a tissue or cover your mouth when sneezing.
Adults can transmit the influenza virus from the day before they start to get symptoms up until 3–7 days after the onset of symptoms. Children, however, can transmit flu for more than 7 days after the symptoms first develop.