I have this thing with forgetting to change out the water in my camelback, so I often end up drinking the water that was left in there from a week before. Is this bad? I'm sure there's some sort of bacteria going on in there, but is it harming anything?

Only you.

I'm not a doctor or even in the health field but .... I often leave water in my Camelbak for months at a time without ill effect. Most of our water is highly treated and contains very little in the form of microorganisms. I wouldn't leave an energy drink in there for long but plain water I'm comfortable with. YMMV

Standing water will breed the back wash of bacteria from your mouth. Standing water is not a good thing especially when you are going to consume it. Next time you go to an ice cream parlor you will notice they keep their dippers in a well that has a constant stream of water. Otherwise the health department would close them down! Granted you wouldn't be keeping ice cream in your camelbak but trust me when I say, standing water is not a good thing as time goes by.

Oh jeez, even my water bottles get skunky inside if they're not rinsed out after a week. I'd recommend remembering to replace that water before each ride!

Just spit out the mosquito larvae and you'll be fine.

you are fine, at worst you would probably get the poops.

Water containers, CamelBaks, canteens, water bottles, etc. I just top off and keep re-using until............well, whenever. Now if it's had anything else in there, besides just water, I'll dump it after I'm finished with it and throughly rinse with hot water.

Toss it in the fridge if you have a hard time remembering to empty it. Now you have to remember to put it in the fridge.

You made it this far.
I refresh each day because I like cold water. I commute and have 13 miles each way. I got to a point that I fill it just enough for my ride in to work and just before I leave I feel it with ice and water. Weekends I do flush it out and let it dry but that’s me. One of my fellow riders only put water in and never cleans it out. I have yet to tell him of the strange growth on the back of his head

It only stays in my bottles/camelbak until I drink it all!
It pisses me off to no end when I have leftover water from a bike ride, etc. I always finish what I'm carrying so I don't have to carry it anymore...

Let us know when you start turning green or some other crazy color.

As far as how long the water will last in your camel bak depends on the enviroment it is in. I'm going to assume it is not in the refrigerator, but also not in a real hot & humid enviroment either. Maybe at the warmest 72 degrees in your home with the pack sitting on a shelf, a counter top or even from your bike. More then likely nothing nasty will grow in it. But I wouldn't continue to chance fate.

I highly recommend you either remember to drain it & hang it up to dry, you can purchase a special drying hanger for it. Or you at least put it in the refrigerator after you take it off, hell if you have to put the whole pack in there if you're to lazy to remove the bladder from the pack.

If your throat has been hurting like you're coming down with a cold & you can not explain why then it is probably from something in your camel bak. Gargle listerene mouthwash & the problem will soon be cured.

I've been using a camel back for 4 years now, I own differant models. After every use I remove the bladder from the pack, refill it with water, wrap the bite valve in a plastic baggie, rubber band the tube to the bladder & put it in my refrigerator. That is where all 4 of them stay until they are used. I have one complete shelf in my frig. just for the camel bak bladders. Once a year, at the beginning of Spring I use the cleansing kit that Camel Bak recommend using & give them all a good scrubbing. I just got finished with that the other day.

I suspect that the water is in a very high humidity environment.

Well I just cleaned out the camel- looks good to go.

Mostly I've just been topping off whatever is left each ride... maybe that's been helping to build up my immunity over the years???

Me too.

odd, mine starts sprouting algae if I leave in in my camelbak for a week.

What model of Camel Bak do you own? Have you purchased the hanger that was designed to fit inside the Camel Bak so you can hang it up to dry? What about the cleaning kit? You can purchase a kit that has the brushes that will scrub the bladder, the bite valve & the drinking tube. The kit also contains the cleansing tablets that you use. Once you run out of the tablets you can purchase more with buying a whole new kit.

Like I said in my previous response either hang it to dry or keep it in the frig. I have never had anything funky or nasty grow in any of my Camel Baks because I do this.

One last thing it is recommended that you replace the bite valve once a year. You'll have to anyway as it will wear out.