DE is not a medication, all it is is fossilized remains of diatoms. Mixing it with the bedding or dusting birds with it may help prevent mites. Should you have an outbreak of mites you will still need to treat with something effective that definitely will kill the bugs. When inhaled DE can be irritating to the respiratory tract. If chickens get sick then yes, depending on what the illness is medications can save their lives. In terms of preventative measures, deworming is a very good idea. Hens will appear healthy for a long time but if they develop a heavy load of worms their health will deteriorate. Left untreated worm infestations can kill so it's much better to stay on top of it before it gets so far as damaging a birds health.