Yes, and they might look for a way, or even make a way, for an excuse to let you go if they discover you have a mental or physical illness and have to take a lot of time off. (Or, they may fire you just because you have a mental illness, but they'll say it's because of poor job performance, or something like that.) They might lower your position or move you lower and lower in the company, try to make your life hell so you will quit, and/or keep/create documentation of your poor work skills and performance. Then it's your word against theirs.