Alright, I read all the responses and only Audrey Z almost mentioned the alternative explanation (and there is one) for SIDS ...
so, first, statistically, the number one risk factor is anyone smoking around the baby (ruins their lungs, so any little problem would be expected to be a big problem). As I recall, the second biggest risk factor was being a boy ...
Anyhow. The alternative explanation for SIDS, which I thought was shockingly logical when I finally found it, is this:
Baby drools or spits up on the mattress.
A specific organism (as I recall a fungus) gets growing (presumably even if you wash the sheets this keeps on in the mattress).
This fungus excretes a chemical that reacts with the flammability chemicals in mattresses, to form a potent neurotoxin *that is heavier than air* ... so it stays within 1/4 or even 1/8 inch of the mattress ...
And babies on their backs, their nose is too high to breathe it in, but babies on their tummies, their nose is right there in it. It's something ridiculously scary, like a paralytic neurotoxin : P ...
anyhow, that was like ten years ago, in a highly alternative newsletter, but it claimed New Zealand started requiring mattresses to be sealed somehow and dropped their SIDS rate enormously ... here, I Googled SIDS "New Zealand" mattresses, one typical search return was
http://www.cure-guide.com/Natural_Health_Newsletter/Baby_...
...
anyhow.
watching my babies, it was pretty obvious that they "start" (startle) because their natural (monkey-type) instinct expects mommy to be Right There ... it is only if they don't encounter resistance they wake up/get scared. I mostly let my kids sleep right up against me (I held them when they were quite small in the crook of my arm while we slept, so their head was elevated, preventing all fluid from leaking up into their ears ... not a SINGLE ear infection in the FOUR of them, which TOTALLY rocked ... I was a kid with lots of ear infections ... ), but I also let them sleep on their tummies sometimes if I was desperate for sleep (until I read the New Zealand/fungi explanation, and then I figured I was good to go because we were on a non-chemicalized futon). I also figured out that if you put a good firm weight across their tummies (and especially their arms!), like a good medium-weight pillow, they would start, and the pillow would push back, and that was enough to fake "mommy's here--I'm safe". Of course, since I co-slept, the pillow trick was what I used when they were napping while I was awake, and as soon as they were strong enough to move the pillow, I stopped (smothering and all that).
The other thing about tummy-sleep, that I forget who told me but it was about my pregnant body, is that someone pointed out that we are bipedal and vertical, but our entire innards are still literally hanging from our backbone ... and it is easier for everything if we let them hang and don't set the weight of them sqaushing their own blood supply (this was during a conversation about causes of fetal distress and fetal oxygen lack during labor--because the uterine blood vessels are being mashed and constricted when women are on their backs ... besides that it hurts like the dickens to be on your back of course ;) sigh ... ! ). Once I was watching for it, all of my babies breathed SO much more easily when they were on their tummies ... also it is important when they get congested, to let the mucous drain out instead of back into the head and chest ...
anyhow. As far as I was concerned, I had found the explanations for breathing problems and SIDS and startle-waking, and after baby one (who essentially I couldn't get to sleep unless she was snuggled against me, and I hadn't figured out these tricks yet) I didn't have too much trouble getting babies to sleep reasonably (given that at 6 weeks they wake several times a night for diapers and feeding) ...
Hope something here helps ... sorry I'm a few days late.
--K. :)