I have the terrible urge to type, “Wait until you get a load of MY test tube!”
Oh well I gave into my urge 
Easy E:
This might get tricky for me to explain.
First off, I’m going to be a TOTAL git, neither HIV nor SIV is a lethal pathogen. However, AIDS can prove fatal but it requires a secondary infection. I’d contrast SIV/HIV to family filoviridae or small pox or the flu. The latter all can cause direct lethal infections.
As for SIV infections being totally benign in all primates, I’m not sure about that. I’ve seen data where SIV infections caused CD4 levels to plummet in infected primates, I’ve worked with highly pathogenic SIV isolates that kill CD4 expressing cells very rapidly.
I’d feel better saying that not all SIV infections result in disease progression.
It has to do with adaption to the host. African Green Monkeys and Sooty Mangabey Monkeys both can be infected for the duration of their lives and not show symptoms of disease and have stable high CD4 counts. In other species, there are different effects–even with virus isolated from those monkeys.
I don’t know very much about the pathogenesis of the various SIV isolates in major species of primate. But I wouldn’t be suprised that is a SIV isolate adapted to a specific speices would have a VERY different course of infection in another species.
There is a lot of research being done trying to find out how and why SIV is less pathogenic than HIV. Mutations within the NEF gene are being heavilly investigated. A few years ago it was found that partial nef knockout mutations would cause a disease progression similar to long-term human non-progressors. They also found that a nef knockout mutant would provide protection against reinfection with a wild-type strain. However, in the last 2 or so years evidence has been found that humans can be infected by multiple strains of HIV.
Which brings up an interesting point which I don’t think gets covered in most public health HIV education. If you’re lucky enough to get infected with a strain of HIV that has the 15-20 year latency period, or you have your infection undercontrol with the drug cocktail therapy you can STILL get infected by another HIV strain.
The second HIV strain might already have multiple drug resistances, or it might be HIV-2 which progresses much faster.
Anyhoo, I digress.
SIV is researched only because of the different pathogenesis in non human primates but also because of the smiliarity in the genes and mechanisms of viral infection and growth. The goal for HIV research isn’t to find a way to change/inhibit the virus so it doesn’t compromise the immunesystem. HIV still has a high mutation rate, and as long as the virus is still in a host it can mutate to regain its pathogenesis or mutate to make the treatment unaffective. We’re already seeing this in the multiple drug resistance strains. We’re trying to find a way to CLEAR the virus. Or failing that, create a vaccine to prevent the next round of people from getting infected.
We’re hoping either for a cure, like with antibiotics and bacterial infections, or for wiping out the disease like we did with small pox and the vaccine.
I’m still rambling a lot mainly because I start typing then think of something interesting to share then I try to get back on track.
I wonder how the monkeys would feel about having glow in the dark genitals. You know someday humans might really want to have glow in the dark genitals too. It may become the fashion rage of the future. You never know. If it does, we can say we heard about it right here on the SDMB. [giggle]