Will ALL diseases inevitably mutate into a sweeping epidemic, given time?

(Apologies in advance if this is the wrong forum.)

When reading on the Ebola outbreak, I keep seeing statements and comments along the lines of “wait until it [or H5N1 or whatever] becomes airborne, or a little less lethal so it can spread more — then we’ll have a REAL problem on our hands!”

That got me thinking: why doesn’t that mean that ANY disease can do the same? Given enough time, especially in today’s modern world, why can’t ANY communicable disease mutate into this perfectly transmittable and fatal global pandemic? Why isn’t the common cold some kind of superdisease by now? If ebola and the flu are special, why?

(Note: I’m not freaking out about this — I actually thought of this as a counterargument against the comments I mention in my first paragraph, and I’m trying to find the flaws.)

It probably could happen in principle with any infectious agent, but there is no particular reason, AFAIK, why it should. Indeed, rather the opposite, it is not really adaptive for a pathogen, especially a virus, that relies on the living cells of its host in order to reproduce, to wipe out, or even seriously reduce the numbers, of their host organisms. Indeed, if all the hosts are wiped out, the pathogen becomes extinct too. The really successful pathogens are ones like the common cold (actually many pathogens with similar symptoms) which keeps their hosts alive and available to be infected (i.e., reproduce more cold viruses) again. Ebola is a rather unsuccessful human pathogen because (up to now) epidemics have failed to spread very far, simply because most of the humans who have got it have died so quickly that they did not pass it on.

The key difference between Ebola or Avian Flu and the Common Cold, as I understand it, is that the Common Cold is a virus that targets humans. Ebola and Avian Flu are diseases whose primary targets are animals.

So as njtt says, it’s in the evolutionary interest of the Common Cold to keep us alive. For Ebola, the fact that it can target us at all is just a fluke of nature and as such Ebola has no reason to be kind to us. All humanity could die and its ability to continue on as a life form would be unaffected, for the most part. The direction that it mutates has nothing to do with optimizing itself for us, so the direction it goes is entirely random and, potentially, devastating to us.

The other issue is that, with Ebola, it’s on the wrong side of the hill. If it wanted to target us, it would need to become less horrific, so as to keep us alive, so that it can better propagate itself. But at the moment, the number of people who die from Ebola is only as low as it is, because the disease is so horrific. It kills itself out. As it lessens in danger, it will actually spread wider and kill more people. Over time, it would presumably lessen even more, to the point where it becomes something like the Common Cold - a drag, but not a killer - but to get there, it has to go through the worst territory for us.

Since it seems the question has been answered already, I hope you’ll forgive the slight hijack, Leaper.

This makes me wonder if some of the common cold viruses, or some other nowadays-mundane disease, was once a horrific pandemic-type pathogen at some point in human history. Presumably some of our more mundane diseases took an interest in us millions of years ago and have been evolving with us, whereas others are relatively recent and had to go through that killer phase. Was a cold virus once what we fear swine flu or avian flu will become?

Do we know much about this?

The Spanish Flu pandemic of 1918 was caused by an Influenza virus that is now circulating as a seasonal flu virus all over the world. In terms of lethality it is not even a shadow of its former self because humans have developed immunity to it (the immune system knows how to recognize and defeat the virus).

Give how much time? Macro-organisms have been around for millions of years, with pathogens free to mutate into a sweeping epidemic to wipe them out, but it never happened.

In fact, it was over-reaction of the immune system that killed. That’s why it was deadlier to young healthy people than to infants or elderly. I assume it mutated to set off a less vigorous immune response.

“Perfectly transmittable” and “fatal” are generally opposites. The quicker it kills you, the less time it has to spread to another host.