Well you can wash it, but as it produces hot steam, that will kill every kind of normal germs.
But even if you do use it with destilled water, after a time there will be some kind of settlings on its ground and you can use vinegar to get them away.
To find the right one, just look inside, if theres enough space for all the parts of the nebulizer and - if you have one- the flutter. Its also good, if it is turning of itsself; you put away the top then and the nebulizer parts will dry fast, because of all the hot steam fading away. If you want to be extra careful, you can dry them with a tissue or something.
Uli,44,Germany