Ebola Reston has shown signs of being transmissible via air. Although it has only been confirmed to infect crab-eating macaques, the potential to jump to a human host is unsettlingly high. Seeing as from a "virus's perspective" a human and a macaque aren't very different. Our cytoplasmic membranes aren't that morphologically divergent, due to our common lineage. Currently it's unknown if it can affect humans, though there is a fair body of evidence suggesting that it can't. However, it could easily mutate to a human hosts; it's basically just a roll of the evolutionary dice.
Ebola Reston actually infects homo sapiens, but it doesn't produce the deadly hemorrhagic fever symptoms. If Ebola Reston acquired the lethality of Ebola Zaire, it would be a highly dangerous disease.
The prospect is worrisome, since in addition to crab-eating macaques (the animal in which the virus was first discovered), the virus had made the jump to pigs as well. Porcine to human transmission is also a very probable species jump. Since there is more human-pig contact that human-monkey contact, there's a dangerous prospect that a virulent air-spreading form of Ebola could jump the species barrier.
If that occurred we'd be faced with a plague as deadly as Ebola Zaire with the airborne transmission mechanism of Ebola Reston.
But why (how) would a virus evolve deadly symptoms if it doesn't already cause deadly symptoms. I always thought that the rate at which Ebola Zaire kills its hosts is actually a hindrance to its further evolution.
In general do virii tend to evolve deadlier or less deadly symptoms?
It would entirely depend on the ecological niche it's exploiting. Ebola's preferred host (currently unknown, though bats are suspected) isn't human. Though its symptoms on humans are catastrophic, it may be asymptomatic to its primary host. It's effects on humans being only coincidental.
Viruses and bacteria don't need to evolve "linearly", as you imagine. If two related viruses infect the same host, they can swap genes and the "airborne capability" gene can jump from E. Reston to E. Zair. The Reston strain doesn't have to "evolve deadlier symptoms".
It's a random process, and the rate of successful recombination is low, but it's the kind of dice you don't wanna let rolling... This is why more developed countries should start to give a fuck, because once we have a successful recombination it becomes everyone's problem!
(Along other lines, you seem to have the same "pretty wrong" idea of what evolution is and how it works that most people have... evolution is not a "directed" process, and it doesn't always work for the benefit of the "organism" or "species", it's a random search, and informationaly speaking it happens at at the genes level, which doesn't seem to mean much at our multi-cell organism level, but for bacteria and viruses it totally changes the game, as the "things that evolve" are actually the genes that not so infrequently jump "horizontally" from one strain to another... once you go down to the "micro" level, the way real world evolution works is pretty different from the "high school biology textbook definition" that most people have of it... and unfortunately and can produce very nasty results.)
EDIT: to make the prev paragraph clearer: if you have a bunch of species/strains that can share genes, then traits that are detrimental to any of the actual species/trains can actually be favored by evolution, if they increase the survival chances not of any species/strain but, but of a "cluster of genes" that manifests a particular feature, and it can be pretty mathematically complex and unintuitive to think of what makes a "cluster of genes" more "fit" (short term "gene cluster" fitness can be compatible with one of the strains "committing ecological suicide" by evolving capabilities that kill its host and limit it's spread in the process) ...the point is that at this level things work very differently than we imagine and we can't just hope that "evolution will work for the best and save our asses"!
EDIT2: this is not just theory, "gene jumping" happens all the time with flu strains and other viruses (for flu it's its main mechanism of evolution - or "the mechanism of evolution of the flu genes cluster"), but it's not dangerous because the viruses are all well adjusted to the human population and vice-versa ...Ebola, on the other hand, is a virus originating from animals, that hasn't had contact with that many human hosts in its evolutionary history, and also a large percent of the current human population hasn't had any recent ancestors that came in contact with this virus, so this is no "good ol' friend" like the flu, it's quite "alien" to modern humans and it can do lots of damage before achieving some sort of "ecological equilibrium" or before being exterminated by a vaccine and/or effective treatment.
