RE: Diseases from a used machine
My (over)education is in microbiology and molecular biology, and I have worked in all sorts of lab environments, including some with rather nasty bugs, so I like to believe I know a thing or two about the topic at hand. That said, I am also relatively new to CPAP and have not worked with bacteria for several years now, so I am not quite up to date on the literature as I used to be.
As far as diseases from a used CPAP, sure it is possible. I would expect rather unlikely, however. We live in a microbial rich world and our bodies are vessels for more bacterial cells than our own (as noted by others), with the majority of those being beneficial. Otherwise we would not have survived this far on earth. I have a rather laid back approach to bacteria, I have to admit. I will eat food that falls to my kitchen floor and I am loathe to take any antimicrobials.
As others more knowledgeable on CPAP have noted, the machines are relatively free of bug friendly surfaces that hold liquid, except for after the humidifier. A good cleaning of the humidifier should solve any worry there. Toss in a disinfectant and you should be OK there. Only thing I would worry about would be the baffle in the (Respironics is all I have experience with) humidifier, if it managed to get get any water into it and sat for too long. For backup I picked up a used 560 model and stripped it down, mostly out of curiosity, but did clean all the internals with some 70% isopropanol just to make sure no smoke or other residue was present, not for the bugs. I could not see far into the humidifier baffle, thanks to its clever design, but did give it a good wash with detergent soap, water rinses, and followed by some isopropanol to dry it out. Then gave it a good several day dry on the counter before reassembly.
Contrary to what others say, I believe there is the possibility of growth on the insides of the air channels of the device (in the last 20 years or so, we have learned life can and will exist pretty much anywhere on the planet, manmade included). The filters are not 100% effective, but effective enough for those of us without compromised immune systems (I did discover both filters of my daily machine popped off one day, I should note). I don't worry about it that much and, as noted, only wiped down the insides of the used 560 to remove dust and smoke, if there (promised smoke free when I bought it off Craig's List but cigarette smoke irritates my lungs). This machine had less than two month's time on it, so I did not expect to find anything, and did not see anything visible on the wipe cloths other than a spattering of dust (which probably did have life, aka bugs, in it).
By bugs, I mean bacteria mostly, but also include fungi, protozoans, and viruses. All have the ability to help and harm us, but for the most part there is a dynamic balance between all. So I suggest stripping down a used instrument if bought somewhere other than the well known second hand vendor, and cleaning it, just to make sure no dust or smoke residue is present. If filthy on the inside, then consider it a learning experience, and toss it. As for bugs, I would not and do not worry.