Can vaporizers be bad for your lungs?

Only if dirty: Vaporizers can breed bacteria and viruses fairly easily if you don't take the time to clean them daily. If that happens, then aerosolizing them in the steam and breathing it in could be a starting ground for pneumonia or bronchitis. As long as you keep the unit clean, you will be fine.

Only: if the water is contaminated .

Possibly: One problem with steam vaporizers is they need to be cleaned regularly. They create a great place for mold, mildew and other germs to grow. If they put those germs back into the air, and you breathe them, that can certainly cause trouble for your lungs. I prefer cool mist humidifiers (but they still need to be cleaned regularly), especially the ultrasonic type.