I usually tell them that it is a systemic disease that can be debilitating and painful and causes widespread inflammation if I come in contact with a germ, allergy or pretty much anything my body doesn't like.

I answer that Lupus is a diseases that means my body at war with itself. Although I don't look sick, and it seems to you that I am lazy, all my organs, glands, blood and sometimes my skin are at war with my immune system to function. To make things worse the meds I take to help fight back make me even more tired and all have side effects. The war never stops, even when I sleep. On my best days my energy level is like yours when you have your worst flu. When I do get out and have fun or do tasks around the house, I pay for it for days afterward.

If people then start saying things like, well you just need to exercise, eat better, blah, blah blah. I just say. " I am fighting to stay alive every single day. Without my self care and my meds I would be dead within 3 or 4 days. I am extremely educated about my disease and most of the time I do everything I can to fight my fight. Please don't judge me on what you can't see and I won't judge you for thinking I should behave the same way a healthy person does."

I have more but each response to that depends on what the attitude is of the person to whom I am speaking.

I like Debraart's response. I saw a t-shirt that said "my disease is not contagious, but your germs could kill me" I do share that when I am around people that don't already know my situation and they are sneezing or coughing.

i would always say "lupus is an autoimmune disease" connective tissue disease then when they ask me what are the symptoms?...

I rarely talk to peeps about something that is sacred to me. It's none of their business.
When someone tells me I look good, I thank them kindly. It's a nice complement when I feel like crap inside.
God bless you for not caring what others say about us behind our backs. It's really none of our business.
LiCx

I say, " It means my immune system doesn't work right and it attacks my body. They symptoms can be different every day, but something always hurts, and no part of the body is immune to being attacked". They usually look at me in horror.