I think mostly it's the opposite of what you describe. Diversity is key here, because if everyone had all those same molecular markers, we'd all be susceptible to the same virus. Mixing things up gives the species as a whole a better chance to survive a bad pandemic, if there are some groups that will be relatively safer (because their cells are harder for the virus to infect, or their immune systems start with proteins that will be better able to adapt to that specific virus, etc.). Certain genotypes and haplotypes confer more (or less) susceptibilty (or protection) to some viruses and this would likely be the case with anything "new" that crossed over from another species. It's all really complicated and of course we're all anthropomorphizing species and viruses and cells and proteins because it makes it easier to understand. Evolution is wild.

On a side note, cheetahs are really lucky. There is an incredibly small amount of genetic diversity in the cheetah population--to the point where I've read that any cheetah can get an organ transplant from any other cheetah. One study gave convincing evidence that the total population at one point was down to something like "no more than 7 individuals"... especially at that extreme bottleneck point, but even up to today, it would seem that they may be more susceptible to an extinction-level cheetah pandemic. Something like that could happen to humans, and things would likely be pretty grim, but our diversity could mean a better shot at existential survival in such a scenario.

I’d need more context on why you think humans aren’t genetically diverse, or what species you think are more so.

Again, viruses are so simple they aren’t even considered to meet all criteria for being alive. They’re in someways more like molecular machines, and are 100-1000 times smaller than a human cell in a single dimension. Cells are 3D, so ^3 them and it’s a million to a billion times less volume. At that level they (and to an extent bacteria) can swap DNA/RNA accidentally with completely unrelated organisms. Viruses are so “good” at this that all species have “junk DNA” that appears to be the virus inserting part of its DNA into ours and getting replicated (some junk DNA may play an important role we haven’t figured out).

I say this with caution - the vast majority of these swaps are completely fatal, or are worse than the original and don’t survive long. However each infection (one not quickly squashed by the immune system) creates many many billions of viruses in a human (this is likely approximately scalable by body mass).

Humans as a species with a very plentiful population are pretty genetically diverse. We’re actually arguably more diverse over time as civilization broadly allows survival of otherwise less optimal people. For example I’ve got a pretty decent brain on me, but I’ve had very poor eyesight from an early age, and while a healthy adult was sick pretty often as a kid. It’s very possible I’d have died or been destitute if I was born even a few hundred years ago.

And you can actually see this evolutionary selection in earlier humans. Humans everywhere adapted more to their environment the harsher and earlier it was.

• Skin color: melanin is a defense against UV radiation, which is more intense the closer to the equator you are. Conversely it limits natural vitamin D production, which skin produces from UV exposure.
• Immunity: the Black Death killed something like 20-30% of Europe’s population. There are certain diseases that Europeans have higher likelihood of resistance than average
• Persistent Lactase: Europeans had access to more domesticated animals that produced milk, and are the least lactose intolerant group.
• Altitude: natives to the Andes in SA have far better tolerance to thinner air at 1-2 miles above sea level.
• Malaria: Africans with great malaria exposure, are at much higher risk of sickle cell anemia, because carrying only 1 copy of the gene produces significant resistance to malaria.

Bottom line is that humans are pretty diverse, and in any case it’s hard to compare genetic diversity of complex species to single celled (or zero celled) organisms.