Absolutely wrong.
Viruses don't become deadly. What is deadly is surprise cross-species contamination. I.E. The Spanish Flu in the 1920's was a Swine Flu that suddenly crossed over into humans. A minor bug in pigs is deadly to humans. For a while.
Then it mutates into a less deadly version AND people become more resistant.
Every single deadly virus ever is cross species.
Spanish Flu - Swine Flu (And other Swine Flues)
Bird Flu - Birds
Bubonic Plague - Rat Disease
Hemorrhagic Fever - Bat Disease
Corona Virus - Bat Disease
HIV - Monkey disease
Rabies - All mammals.
Lhyme Disease - Deer
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1. Not all deadly viruses come from cross-species jumps
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Smallpox was a strictly human virus for thousands of years and killed hundreds of millions. It did not require an animal reservoir to stay deadly.
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Measles likely originated from rinderpest (a cattle virus) centuries ago, but for recorded history it has been a purely human pathogen and continued to cause enormous mortality until vaccination.
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Poliovirus is also human-specific and has caused crippling disease and death without any animal “surprise” origin.
So the blanket claim “every deadly virus ever is cross species” is simply wrong.
2. Some examples in the list are not viruses at all
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Bubonic plague is caused by the bacterium Yersinia pestis, not a virus.
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Lyme disease is caused by the bacterium Borrelia burgdorferi, not a virus.
This shows the argument confuses “infectious diseases” broadly with “viruses.”
3. Virulence does not always decrease
It’s a misconception that viruses “always” evolve toward being milder. Evolution doesn’t have that direction — instead, viruses adapt for transmission. Sometimes that means lower virulence (Ebola outbreaks often “burn out” quickly because killing hosts too fast hurts spread), but other times increased virulence helps transmission (myxoma virus in Australian rabbits initially became more lethal before stabilizing).
✅ In short:
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Not all deadly viruses are cross-species.
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Some listed examples aren’t viruses at all.
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Viruses don’t always evolve to be less deadly; virulence changes depend on transmission dynamics.
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Human adaptation (immunity, medicine) plays as big a role as viral mutation.
Would you like me to make you a side-by-side corrected list of the examples you mentioned (Spanish flu, rabies, HIV, etc.) showing which are true viral zoonoses, which are bacteria, and which are human-only pathogens? That might make the differences very clear.