The flu is caused by a virus. Viruses are the smallest microbes we are aware of. There are four different types of viruses that cause influenza, or 'the flu', as it is more commonly known. In a burst of outstanding creativity, researchers named these influenza viruses A, B, C and D. Mammals and birds host these viruses. Although humans can get the flu from types A, B and C, types A and B are by far the most common. Incidentally, all types of flu are found all over the world.
Recognition and elimination by the immune system
The influenza virus can enter our body via direct contact with someone who also has the virus or because the virus hitches a ride on aerosols (small drops of water in the air) that we breathe in. Normally, our immune system recognises the virus and produces antibodies to eliminate it. This recognition can be compared with the lock and key principle. Viruses do not have cell membranes like other organisms, but they do have a so-called capsin, or protein shell, that holds their DNA. On this shell, viruses carry key-like substances that allow them to adhere to the host cell and then penetrate the host cell. Our immune system recognises these key-like substances and produces antibodies accordingly. The antibodies bind to them like a lock, after which our immune system can eliminate the viruses. In general, it takes about a week from the time of the first contact to eliminate a virus.
Extra help against mutating viruses
Very young or old people, or people with a weakened immune system, can be more seriously affected by the virus. For this purpose, a vaccination was been developed, now known as the flu shot. Vaccines consist of attenuated viruses, or fragments of a virus. In our bodies, this triggers the natural defensive response. The development of the annual flu shot is a complicated business. It is coordinated by a global network of collaborating research centres (WHO Collaborating Centers for Human Influenza Surveillance). The reason the flu shot is required annually is because influenza viruses are very sloppy when it comes to replicating themselves. This may sound strange, but precisely because they replicate in a quick and sloppy way, every time a virus replicates itself there is a relatively high chance that the new virus will be just slightly different from its predecessor. The key substances that allow our immune system to recognise the viruses are therefore also slightly different. And this is precisely why this year's flu viruses are no longer the same as last year's. Every year, the major challenge for the research centres is to try to predict which ‘variants’ of the influenza virus are likely to cause the flu in a few months' time. In order to spread the chances, the flu shot often consists of three or four variants these days.