[cryoshon]

to answer your question, the overhead is very small for a small change. you can add a bit of noncoding DNA to a virus' genome without ruining its ability to compete in the wild. but the survival margins are very thin. on a population scale, natural selection is very harsh. anything that is superfluous given the environment is an inefficiency which eventually results in extinction. of course, between organisms this isn't that frightening because there are different niches, so sometimes a large change can be more viable than a small change even if it's a lot more expensive, provided that the large change lets the organism live in a new niche.

making HIV airborne isn't a simple change, however. it's more like a massive change of niche. it's a change in the transmission modality of the virus -- for comparison, consider the scale of the changes you'd need to make to turn a car into a plane. or maybe a car into a boat.

it's doable, artificially. but the result won't be as good at being a car, plane, or boat as something which was purpose-built for that application and didn't have to carry the features of something intended for a different purpose.

many of the "small" changes that make a disease spread more easily are actually mutations which don't change the ability of the disease to weather external conditions, but rather change the ability of the disease to survive first contact with the host's immune system.

the flu is a great example here. we need a new flu vaccine every year because the flu mutates constantly and drastically. the flu never becomes capable of surviving outside a host for longer than before, though. it just becomes more effective at evading the immune systems of most hosts.

[mikeash]

What about starting with the flu and giving it an HIV-like ability to wreck your immune system? Is the “attack” part too intertwined with everything else to be able to do that sort of mix-and-match operation?

[cryoshon]

the flu gaining deadlier characteristics via engineering is more realistic. unfortunately, i believe that is well within the scope of our present capability. the exact magnitude of how dangerous such engineering could make a virus based on the flu is unclear to me, but i'd estimate somewhere between "globally apocalyptic" and "continentally destabilizing".

the mixing and matching of attack characteristics is probably possible under certain circumstances, but i don't know of any specific instances where it has been done. theoretically, it's easy to swap A for B, but making such changes nearly always has unintended downstream problems.

in the lab we used to do all sorts of mixing and matching, but for defensive characteristics (mostly to see if certain isomorphs were more vulnerable than others).

long story short, generating virus and isolating it is a real PITA for a slew of reasons. experimental cycles might be as long as a week for each trial of "mixing and matching".

[mikeash]

How comforting! Thanks for all the info.