It's not just virulence-it's a bit more complicated. Example HIV is (without current chronically given drugs) very deadly. One of the things that makes it so successful is the lag between transmission to and death of a patient. The virus being transmissible for subsequent spread during that time, which could often be a decade even back before modern treatments (80's or 90's).

makes me wonder if there are viruses that improve lifespan.

Phages: https://www.wsj.com/articles/genetically-engineered-viruses-...

Cool concept. There are examples of behavior change of the hosts by virues and other: https://en.m.wikipedia.org/wiki/Behavior-altering_parasite

Not an epidemiologist, but I understand it mostly has to do with the means by which the virus is transmitted. Ebola is very lethal but requires physical contact of bodily fluids for transmission, which contains a lot its spreading. You can probably sit on a plane next to a contaminated person and you will be fine. That contains a lot the contagion vs a virus like the flu.

During the 2014 Ebola Outbreak in West Africa there were numerous people who appeared to have been infected while keeping their distance. Everybody was perplexed and assumed the victims--some of whom were medical professionals--unwittingly came into close contact with infected fluids while caretaking.

A small group of scientists started to speak up in various forums arguing that the original scientific experiments (1960s?) which calculated things like droplet sizes, distances traveled, times to evaporation, etc--the numbers used to determine how far particular kinds of virii can travel and how long they can persist in the environment--was irredeemably wrong. These bad metrics were and still are used to build transmissibility models.

According to more modern metrics and models, while not airborne as its strictly defined, Ebola virus can transmit much more readily and further than what is currently taught. TL;DR: aerosolized droplets can carry the Ebola virus much further and longer than believed. Bifurcating virus types into "airborne" and "not airborne" is dangerously simplistic as infected aerosols can have risk profiles closer to that of traditional airborne virus types, particularly for a virus like Ebola.

I can't find the original article I read--an article by the researcher who, IIRC, was the first or one of the first to empirically show the critical flaws in the original experimental results that current models rely upon. But this paper seems to have a good discussion of the issues: "Transmission of Ebola Viruses: What We Know and What We Do Not Know", https://mbio.asm.org/content/6/2/e00137-15

EDIT: The original article I read: http://www.cidrap.umn.edu/news-perspective/2014/09/commentar...

Transmissibility depends on strain, based on the book Hot Zone. Ebola Zaire was transmitted only through fluids, while Ebola Reston was airborne.

Though maybe the differences are by-products of the poor models mentioned in another comment.

Hot Zone is...not a good book.

> The problem with leaky vaccines, Read says, is that they enable pathogens to replicate unchecked while also protecting hosts from illness and death, thereby removing the costs associated with increased virulence.

https://www.quantamagazine.org/how-vaccines-can-drive-pathog...

NB: I thought I stumbled upon the above article--or one like it--through HN within the past few days, but I can't find the post. Maybe I found it following some other article link. shrug

This is a recurring pattern right? Like after article one is posted a few days later somebody went through an interesting rabbit hole and shares his new found knowledge with a new link. Or is this a second order effect from an echo chamber?

Modern medicine also means earlier detection and quarantine, so by the same logic it should mean less pronounced symptoms so a virus could hide longer.

Deadly viruses are not necessarily less contagious. Contagiousness has to do with a lot of things, including the duration you're able to transmit it (which is sometimes related to virulence because you're dead), the mode of transmission, and the setting its in.

If everything else were to remain the same; keeping the infected alive longer wouldn't necessarily do much at all. The trouble comes when that person spreads the virus to someone else, perhaps with a compromised immune system. If the virus load in the newly infected person is the product of that portion of the original virus which wasn't killed by the "99% effective" methods of killing them, then that victim now has a more robust form of the virus, likely harder to treat.

But not necessarily more virulent.

But if you can keep them alive the virus is somewhat mitigated. It may make the virus more pervasive, but less deadly.

They probably look like the flu, which mutates rapidly, with the virulence and symptoms drifting around.

The rapid mutation is why annual vaccinations are necessary.