If Covid-19 were a killer virus with zero chance of survival it would have been possible to contain it because doctors would be able to identify all infected people and separate them from others. The fact that there are many people who don't develop serious symptoms or are even asymptomatic, makes Covid-19 so hard to contain.

There are already many diseases with 100% fatality rate or a fatality rate over 80%: <a href="https://en.wikipedia.org/wiki/List_of_human_disease_case_fatality_rates" rel="noopener noreferrer" target="_blank">https://en.wikipedia.org/wiki/List_of_human_disease_case_fatality_rates</a>;. Rabies is caused by a virus. Untreated it's almost always fatal. Creutzfeld-Jakob is caused by prions. It's uncurable and within one year 70% of infected people die, so far nobody has survived longer than 2.5 years, so it has a 100% fatality rate. Ebola has a fatality rate of 83 to 90% if untreated. HIV is uncurable but nowadays it can be treated well. If untreated it has a mortality rate of 80-90%. When HIV was detected, it definitely felt like that it's a killer virus. But as it can be treated now, many people don't take it seriously anymore.

Imagine COVID-19 mutated and become 100% fatal. Same symptoms, same airborne contagiousness, same rate of spreading, except it always gets to the lung and destroying them with zero chance of recovery. Hospitals would be overburden since every case of the disease destroys the lungs and requires hospitalization. But breathing machines would be useless they just buys couple weeks of miserable existence being attached to those machines with zero chance of recovery.

Eventually healthcare system collapses completely because doctors would be dying and quitting en masse since everyone succumbs to the disease regardless of their efforts. The smell of death is inside most of the houses because most of the families are dying inside their flats since there are no functional hospitals anymore and not enough cleaning teams to deal with the rotting bodies inside the flats. At this point government realizes there would be no "herd immunity" every single person will just die so it implements way more strict quarantine measures than you can imagine. It kinda slows the virus down. However, in such strict quarantine the economy cannot function as usual. Either people would just run out of stuff and die from starvation or from the disease if quarantine measures slightly relaxed.

In such situation I see two scenarios. First, electricity is no longer produced and grid is no longer maintained, water and natural gas are no longer pumped into the houses, there is no food, society completely collapses and in this chaos disease kills everyone except few who managed to escape to private islands or locked inside secret vaults. In second scenario I see government managed to establish totalitarian super surveillance state and total control over all its citizens movements and contacts where commanded economy is implemented.

Let's just image if this virus was like the Black Death. And even if the doctor was going to discover a cure and how to protect against it, at least billion people or more had been died. It would be like the movie called "Flu" on Netflix, but instead of happening in one city, it would be happening around the world. And the vírus was going to pass from one person to another very faster than COVID19.

The economic problems would increase other types of deaths, since people hadn't any condition to survive without a work and to keep their families in a security place. That would be a devastating time to everyone.

I think if the virus was mortal, it will not spread so fast like covid-19 becouse people would die very fast and so they will not have so much time to pass the virus to another person.

@Raphael

Some movies like "Elysium" and the TV series "Colony" are similar to what is happening, or what will happen in the near future as I guess.