I have a benign form of multiple sclerosis (MS). I'm fortunate, because I won't end up in a wheelchair or on a respirator because of it, but it does cause some wonkiness in my brain function. Particularly aphasia. From dictionary.reference.com: Loss of the power of speech, or of the appropriate use of words, the vocal organs remaining intact, and the intelligence being preserved. It is dependent on injury or disease of the brain.
Basically, my brain works like a wonky IBM PC (the original IBM PC from the early 80's). You know how your computer all of sudden just stops working and the screen freezes up and you can't do anything except hit ctrl-alt-del to get it unstuck. My brain does that all the time. Or you're on the net and you get a server error can not connect to the website. My brain does that, too, all the time. So, I misspell words and leave words out and use the wrong word. Homonyms are the bane of my existence. Yes, I know the correct usage of there, their, and they're. But, I type the wrong one all the time. Sometimes, I catch it. Sometimes, I don't. Sometimes, I don't realize until I reread something I wrote a few days later. Proofreading doesn't help as I do proofread everything that I post.
So, how I plan to handle this little bit of nuisance is that I have someone I am paying who will edit and proofread the publications to catch all those typos, missing words, and the like which I do not always see.