That's not true at all. There's no reason something can't be more lethal and more virulent.

Ebola is spread by contact, not in the air, so it's not a good comparison. (Especially because it can spread very quickly even given that it doesn't spread through the air.)

Many experts accept that viruses evolve to be less lethal, and more infectious.

  “I believe that viruses tend to become less pathogenic,” says Burtram Fielding, a coronavirologist at the University of the Western Cape, South Africa. “The ultimate aim of a pathogen is to reproduce, to make more of itself. Any pathogen that kills the host too fast will not give itself enough time to reproduce.” [1] 

That same article though says it's tough to generalize evolution. It's possible something could evolve to be more lethal and more infectious. However, that's not in the best interests of maximizing the survival and reproduction of the virus, and that is what evolution typically selects for.

[1] https://www.smithsonianmag.com/science-nature/how-viruses-ev...

Just because they tend to move towards less lethal doesn’t mean they never can go the other way, as evolution is not a plan or choice. Something increases lethality but extends overall illness time, increases transmissibility, increases asymptomatic time, or a combination would outcompete other less lethal versions. While, in the long run, a less lethal virus would be more beneficial to the virus, that wouldn’t stop a more lethal version from burning through a population until it dies out by destroying its own habitat.

How long will that take? One of the excuses last summer from many Covid downplayers was the exact same sentiments and then along comes the Delta variant in October 2020.

There is no hard and fast rule that a virus will evolve to be less lethal or/and less infectious, just a noticeable downward trend in other instances. Well I'm still waiting for a downward trend even with the vaccination efforts.

What about viruses that are designed rather than evolved?

Why the downvotes? This is a really important question given just how easy it already is to design a virus.

>There's no reason something can't be more lethal and more virulent.

Smallpox.

>What about viruses that are designed rather than evolved?

Biopreparat(tm) Smallpox.

It would be much easier to identity an infected individual for quarantine if a virus had a steep fatality rate, the transmission would be much slower too since dead people don't travel.

Only if the virus makes its presence known quickly. Something can be fatal, yet take months or years to kill you.

Worst case would be a highly lethal, airborne virus with a long delay between infection and death.

Viruses tend not to work like that because to effectively spread via respiratory droplets and aerosols they require a high viral load in the respiratory system. This generally causes symptoms.

I’m sure it’s not completely impossible, but neither is a massive radiation burst from a nearby supernova.

Unfortunately, this isn’t true. SARS-COV-2 actually gives us a model of how a virus can be both.

- Pre-symptomatic transmission

- Early immune evasion and suppression

- Strong inflammatory/cytokine response