I live in a humid area and I still haven't noticed anything icky looking or smelling in my humidifier or equipment at all. I only use distilled water. I've gone months just topping my humidifier up every night (or couple nights) and not cleaning it. And when I have cleaned it, it's just because I felt like I ought to, not because it had started growing mold.
When I thought about it mathematically, it made sense. Let's assume 1/3rd (roughly 33%) of the water is used every night, and that it's used evenly. Physics is not my strong point, so I don't know if some water would tend to just sit on the bottom and not be used? But the water does get jostled around when I take the humidifier out to refill it. And if it was just sitting on the bottom, not being exposed to anything, I don't know if bacteria or fungi would grow there to begin with?
On night 1, I fill the machine. So it's 100% of this "night 0" original water. I sleep, I use up 33% of the water.
On night 2, I top it off, so it's 66% "night 0" water and 33% new water.
Night 3 has about 43% of the night 0 water (0.33^2 * 100 = 43.56)
Night 4 has 29%.
... and so on. In a week, there's only about 5.5% of the oldest water.
In two weeks, 0.3%. Obviously it only gets more and more diluted from there.
So even when I went months without completely emptying my humidifier, there probably wasn't any months-old water in there. The vast majority of it would be less than a week old. Exponential decay is a powerful thing. (Of course, there will also be some amount of the "intermediate" water, that isn't the oldest or the newest. But all of that is also being diluted every night. You won't have much of the water from night 2 by then either.)
And again, this is distilled water, mostly not in contact with the air, and in a container that I thoroughly cleaned before filling.
I don't wash my tubing, I just replace it. I tried washing my tubing once, and I felt like I had probably exposed it to more bacteria (+ etc) in the process than if I'd just left it. Like it's not getting tap water near it, which is good... and now I'm putting tap water in it? Tap water that doesn't feel like it's dry even if I have my machine blow 20 cm H2O air pressure through it for an hour?
I should clean my mask more, but I'm concerned about the seal more than anything. And acne.
I've been using CPAP for 3 years and I still rarely get sick. I did have one cold that I went to a doctor about because I was coughing for awhile. From what I/they could tell, it was just bad luck with a virus. They didn't give me antibiotics, which was the right decision, because I got better on my own not long after. Anecdotes are not data and all that, but still...