Cold or flu?
People sometimes confuse these very different viruses
At the root of most runny noses are one of two different kinds of virus: orthomyxoviruses (influenza) or rhinoviruses (the common cold). There are many different strains of each, but influenza viruses generally cause much more severe symptoms.
Viruses evolve very quickly, meaning that under the selective pressure of our immune systems, new cold and flu strains can emerge within a single season. Viruses have only a few genes, which means that one mutation can make a big difference. It’s this ability to adapt, known as antigenic variation, that produces viruses that can dodge our defences. The immune system might produce antibodies to deal with one strain, but when a new strain emerges, it isn’t recognised.
Flu can be fatal – the 1918–19 pandemic killed more people than World War I – so it is the subject of intense research. Much of this research focuses on variation in H (haemagglutinin) and N (neuraminidase) proteins embedded in the virus’s outer coat. Changes to these proteins make it harder for the host immune system to recognise the virus.
Rhinoviruses, though less deadly, are also remarkably adept at avoiding the human immune system. Some dampen immune responses by interfering with the signals that attract immune cells, while those that receive a strong immune response seem to evolve even more rapidly to escape it. (See our special issue on influenza for more.)Lead image: