We live in an age where there are dozens of labs in almost every country dedicated almost exclusively to mapping new strains of EVERY decease we know of. You simply can't develop a new strain of any decease today and keep it a secret to most populations for too long. Just look at SARS or even Swine Flu. How long do you think it took between the appearance of any of these and it's detection? Weeks? Months? If AIDS were to become airborne by the time it did, it would have already A) been detected by scientists in the particular country or B) have found it's way to a country that would have the means to detect it.
There are programs being run in Africa and pretty much every other country to detect the transfer of AIDS from parents to newborns, programs dedicated to finding out the rates of infected among children, new infection rates etc. Do you think if airborne AIDS were to show up tomorrow by the weeks end we wouldn't already have at least one case of somebody who didn't have AIDS on Saturday morning but did on Sunday night without any exchange of bodily fluids? The claim that we shouldn't let AIDS patients in because of the possibility of an airborne version of the virus simply has no real merit.
Who said it was?It's not impossible that other strains exist, that we don't know if.