And? The terms are something like endemic, epidemic and pandemic as I vaguely recall from some science class in my past educational track. I would have to google [and I am on my phone reclining in bed and I can not be arsed to get onto my laptop and make it easy] that describe how fast and how far/many the illness is spreading or has spread.
The influenza of whatever type is an excellent example. Screw specific strains, to be blunt influenza is actually a shit example.
Going from memory, endemic is how prevalent something is in a region [um, malaria is endemic in for sake of argument Nigeria as a random country name] which means that if you go there, it is in general present in the area and to take precautions. Back in the day, I got 2 sets of shots when I was a shot training dummy for mrAru’s EMS class for the Navy - the recruit package and the overseas package [which at the time also included black death =) as a medieval hobbyist that tickled my fancy] because the places ones military service might lead overseas would potentially send one into areas where those odd diseases might be present.
Epidemic is what happens when it is flu season and suddenly tons of people show up with the flu, or something like half the kids in school show up with the measles. A quick upswing in the number of cases and more or less locally.
Pandemic - pan = everywhere. A sudden upswing in cases everywhere turns into a serious upswing in numbers of cases everywhere. What we just had. We are probably seeing covid go from pandemic to endemic, a chronic low level presence all the time of cases of covid being found. Or, the equivalent of influenze not specifically spanish but in general the one or 2 variants my flu vax has every fall. We will probably get a booster every fall of whatever the prevalent 2 variants are every year. [or maybe if someone finds a universal covid vax that]