Where do pandemics come from?

Tara, it's been ten years since I read Paul Ewald's "Evolution of Infectious Diseases" and I'm curious about the current thinking about emerging infectious diseases in that context.

I'm aware that both of the main hypothesis that Ewald puts forward in this context — the 1918 flu pandemic and HIV — have been disproven. This is damaging to Ewald's argument, of course, but it's also perhaps unfortunate that he went out on a limb, given that the essential argument — pathogens will evolve toward increasing virulence under certain conditions — seems to be moderately well-established in more narrow contexts.

So the consensus was, and remains, that highly virulent pathogens and the occasional resulting pandemics are generally zoonotic. But it's still possible, isn't it, that an endemic pathogen could take a strong turn toward virulence if the ecological conditions were dramatically altered — specifically, that a reliance upon an ambulatory, less-virulent direct contagion would be superceded by a new external vector, lessening the selection against virulence?

Is there new, important work in the evolution of pathogens, or has this (the work of Ewald's et alia) been refuted or just dormant?

I'm also a bit rusty in this area, but at least since the last time I looked into it maybe a year ago, I think largely just dormant. Unfortunately there seem to be many exceptions to the "rules" Ewald put forth, so it's been difficult to really pin down general trends in the evolution of virulence. I like his underlying ideas and they seem to make sense, but biology is just so messy it seems that for every "rhinovirus evolves to avirulence" you get a "yeah, except in this case..." scenario.

Thanks. I'm just an (informed) layperson, but I was very impressed with Ewald's book and I agree with you that his ideas make a lot of sense. The two different directions — epidemiology and clinical evolutionary medicine — both seemed to me to be full of promise, accounting for something that had in the past been overlooked and which can sometimes be of crucial importance. The case studies he presents about the neonatal units still loom large in my mind, ten years since I read the book.

Tara - I posted this to Orac, to get some scientists involved in the question. Maybe I'm misreading it, although the comments seem to make my reading sure, but people seem to be arguing that it is better for HIV+ people to conceal their condition and have unprotected sex then for them to reveal it and possibly suffer prejudice or discrimination. That's how a lot of us read it, but it would be nice to hear the perspective of someone who is familiar with infectious diseases.

http://freethoughtblogs.com/crommunist/2012/12/05/hivaids-stigma-canadi…

One example I coud think of a pathogen turning MORE pathogenic due to changes in environment is the recent investigation into salmonella evolution in the age of HIV http://www.newscientist.com/article/dn22318-hiv-could-be-turning-salmon…