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So what is a pandemic anyway?

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’.