Anything evil is described as "devil", as an adjective used for anything that harms anyone.
As you know quran has the same massage of taith as in's masi7's bible, musa's tawrat, adam's and all the other prophetss sent before, each with a spesific style to fit for a specific era, as to excute justice.
However, when the past eras ended, those messages got corrupted, becauae humans forgot it's basics and princples, got lost with the aspect, they started misunderstanding it and corrupt the origional moralities, they replaced them with stupid superstitions and so on.
One of the terms which got mistakingly used was "devil". Now adays people think it's a kind of creation, like humans, which is entierly incorrect. In Quran, Allah reminds humans, corrects this like most of other things. Showing that humans can be devil too, meaning devil is a term, a characteristic to represent an evil person/thing.
In Islam hygien rules, devil can be bacterias, however all what can a devil do is to make you forget, forget faith, it's just a distraction, it's not even a threat.
Hallucinations, stress, sickness are all devil, it makes you have less energy, thus you wont be positive to achive basic levels of faith, imagen a city with no foundation.
However it is true that "jin" or "genes" exist, we don't see the they see us, they can be faithful or devil (evil), they can be harmful or harmless, but again nothing affects a faithful person.
You can clearly see why if you consider "chakras".
Just follow islam and you'll be surely protected.
Now, the memory loss thing isn't even a problem when Quran is a "thikr" meaning it's a reminder, when you read it, you'll remember.
I do use that, whenever I forgot what I studied during tests, I say "bism-el-la" it means "In the name of allah" this is what you have to say at the begining of every verse.
Once you follow Islam you wont have enemies. With out it humans are lost.
The prophet muhammad (peace upon him and all the nessengers) described himself like a man calling people who are attracted to the fire like bugs. You're shown the path, the right way, and the truth.
I realised I used a lot of "and". I don't want to go through it and edit it, it came to me like that, because that's what we all know.