The terms endemic, epidemic and pandemic are used to describe the way diseases (not just infections) affect populations of humans or animals. The terms epidemic and pandemic have a very similar meaning: an epidemic is a rise in the number of new cases of a disease in a population; a pandemic is simply an epidemic that occurs over a wide geographical area. The point at which an epidemic becomes a pandemic is the point at which experts start calling it a pandemic. Not all epidemics become pandemics. Some epidemics are limited by geographical boundaries especially in remote rural population who do not travel much, or they just peter out.
When a pandemic or epidemic dies down (they always do) the disease may become endemic. This means that it is always present in the population, usually causing milder disease in most people. This endemic situation is what we experience with seasonal flu, coughs and colds, sore throats, viral gastroenteritis etc. The disease fluctuates and people may notice that ‘there is a lot of it about’ but it never really causes a true epidemic – it just grumbles on year after year.
So why don’t pandemics just go on and on? Humans, like other animals, become immune to the infection. When you have a sore throat you will notice that your ‘glands are up’. If you feel your neck at the corner of your jaw, you will notice a marble-sized hard structure on either side. These are your submandibular lymph nodes (or lymph glands) and they are full of B cells that respond to any foreign material (such as a virus) by producing proteins called antibodies that attach to the intruder and allow the body to get rid of it. When you are fighting an infection the glands become swollen and painful, but the best bit is that the B cells remember the bug and the next time it comes along they are ready to produce a big surge of antibodies to kill it.
This is what being immune is. The antibodies are particular to the bug that provoked them, so being immune to one virus does not mean you are immune to another. Vaccines work by injecting a dead or a live infection that stimulates immunity, but not disease, and so makes the B cells get ready for the real thing. Only twice have vaccines been used to eradicate a disease for ever: smallpox and the cattle plague known as rinderpest. However, for most diseases the bacterium or virus lives in the population, with there being enough immunity in the population (through vaccination or natural infection) for it to be held in check with relatively mild disease in a small number of people. Scientists have borrowed the veterinary term for this state and refer to it as herd immunity.
What does all this mean for Covid-19? The virus is a member of a common family of viruses called coronaviruses. This newly discovered virus is called SARS-cov-2. That seems complicated but it isn’t: ‘SARS’ stands for severe acute respiratory syndrome, ‘cov’ stands for coronavirus, and as this is the second coronavirus to cause a severe acute respiratory syndrome: SARS-cov-2. That is the name of the virus, but the name of the disease it causes is Covid-19 (Coronavirus disease 2019). Why the pandemic? Well, one theory is that SARS-cov-2 was endemic in animals not causing any trouble, but it jumped across to humans and found it could multiply and spread very rapidly in human tissues. As this is a new virus for humans our lymph glands are not prepared and the virus was able to multiply quickly and spread, sometimes without causing disease. So it started as an epidemic in China, and quickly became a pandemic.
As herd immunity grows and especially when we develop a vaccine then Covid-19 will hopefully become endemic in the world population. In this future state, occasional people who are already unwell will become seriously ill but, if it does become endemic in the population, mostly it will be just another thing we notice in winter as we cough in a bus queue and remark that ‘there is a lot of it about’.