QFT wrote:
kraftiekortie wrote:
This sort of thing.....animal to human transmission causing epidemics and pandemics—has been happening for centuries.
I was asking why didn't
this particular one get transmitted till last year.[/quote
There are billions of animal viruses out there, and they are mutating all the time. It is extremely difficult for them to jump to humans, it requires both 1) the right mutations and 2) the right proximity to humans.
Imagine an alien came to Earth and had sex with a human. Would they get HIV? Probably not, because most humans do not have HIV. Even if they did, the alien might not be able to be infected with HIV because the virus is expecting human-like cells and doesn’t get them. Perhaps, by chance, eventually the alien has sex with someone who is infected with exactly the wrong strain of HIV - but in that time the alien has probably already got chlamydia a few times, maybe gonorrhoea, and has avoided getting HIV on many occasions.
(HIV is not the best analogy because it has a very high rate of mutation - more using it because people understand the prevalence)