Let's see ... Combine swine flue, bird flu, Ebola, Necrotizing fasciitis, and yellow fever ... we could get a disease that lives in birds, is transmitted by mosquitoes and human to human contact, the first symptom is when your eyes begin to bleed and then you get diarrhea and a debilitating fever, your skin dissolves. Then cough up a lung and die of pneumonia.

It is interesting game to dream up a combination but, seeing as that all of these diseases have been around in some form for thousands of years and they haven't recombined yet tells me that the odds are low. The worrying aspect of this is that humans are invading environments that we haven't had much experience and getting exposed to diseases which were once isolated to small populations of animals in remote locations.

HIV is thought to have originated as a virus that had adapted to monkeys and was benign withing that population. That war, and famine drove populations to go deeper into the forests to hunt monkeys for meat. That during skinning or processing the infected monkey blood got into an open wound. That this got into the local human population where it spread.

The combination of pressures for scarce resources forcing people into remote locations as the human population grows and our use of rapid world-wide travel combine to dig up previously unknown diseases and rapidly stir them into the pot.

As I understand it exchange of genetic information between diseases is possible but fairly rare. The much more common event is exchange between strains of rapidly resorting diseases. Influenza has one of the fastest resort rate of any disease. It routinely changes every few months and exchanges information between strains as a matter of course.

The good news is that these modifications are not directed. They are a random process. The latest swine flu looks to be a result of recent interactions of two strains of swine flu and this is very communicable but not very deadly.

The bad news is that if this swine flu recombines with the existing, and still present bird flu, and the random recombination results in a virus that is as communicable as the present swine flu and as deadly as the existing bird flu, which kills about half of the people who get it, we could be in for a rough time. Being a random process this may never happen. Or it may have happened yesterday.

In the end everyone dies.
http://www.theonion.com/content/node/39236

The bright spot is that even give everything going wrong the human race is not going to be wiped out. Because, like the various diseases, our immune defense systems are adapting, learning.