Viruses (viri?) mutate and evolve at an astounding rate. So what distinguishing characteristics remain that make a mutant flu virus still identifiable as a flu virus and not some totally new and different type of virus?
If viruses were mammels, would the influenza family be all great apes, just humans, just white European stock, just Cletus Brewer’s clan, which all have 11 toes but you can tell the individuals apart? Or just clones of Cletus himself, all with crossed eyes and buck teeth? If such an analogy can’t be made, then explaining why would probably help my understanding.
The flu virus is a very specific virus - one “species”, though using that word with virii brings up all sorts of other problems. And actually there may be a small handful of species that would qualify, and there are others that cause flu-like symptoms without being flu, but that’s not important right now.
The influenza virus is a specific type of virus, with specific characteristics like shape, genetic makeup, method of spread, etc, etc. It is distinctly different from cold viruses, polio, yadda yadda yadda. What makes it different, and complicated, is that the genome consists of, IIRC, eight separate strands of RNA. As viruses spread and grow, they pick up mutations in their genome like everything else does, and that’s one way of getting new strains. However, when two strains of the virus infect the same animal, suddenly these 8 strands can mix and match. You get viruses coming out that have genes from strain A and genes from strain B, and these new combinations can quickly give rise to entirely new strains that are quite different from what we’ve seen before. This is why you’ll hear that the current flu is mostly from a pig strain, but with elements from bird and human flus.
Big scary pandemics occur when you happen to get a combination that is both highly contagious and highly lethal. With your normal genetic mutation-driven drift, it’s hard for something like that to pop up spontaneously. With this tricky mix and match mechanism, it’s all too possible.
Influenza refers to an illness caused by a family (not a species) of RNA viruses-- Orthomyxoviridae.
Symptoms are protean, but typically include a variety of upper (and in more severe cases, lower) respiratory tract manifestations.
Each “strain” of an influenza virus has its own unique characteristics; what makes a virus an influenza virus is that its sttructure causes it to be classified into an Orthomyxoviridae genus (I think there are about 5 of them). This is a technical taxonomy way over my head, but an influenza virus belongs to the family Orthomyxoviridae in the same way great apes (including humans) belong to the family Hominidae.