Er, if you have an autoimmune disease then your immune system should be higher than normal. It is the immune system attacking your own body.
This is why they are often treated with drugs that reduce immunity.
OP
Human immunity is a complex thing. We all have a slightly different system that has white blood cells, various chemicals and antibodies.
Eg if your body encounters HIV the virus here is a 2 stage process for you to become infected, in some people the second process cannot happen so they are immune to HIV.
It is like the white blood cells have locks and the virus has the key, but in some people there is no lock.
When we encounter some viruses our body reacts by creating antibodies, for a lot of viruses this means you are unlikely to get the disease because the body has a model of which antibodies to manufacture and fight of the disease, this is how vaccination works and why most viruses you only catch once (colds and flu change frequently which is why you need a flu jab every year).
As well as your own immune system there is a question of what you are encountering in terms of bacteria, viruses etc. If you are in a completely sterile environment you will not get ill.
Scientists at the South Pole don't get colds because it is too cold for the virus to reproduce.
When I worked for the NHS when I moved hospitals I would get a sort of mild cold for a couple of weeks and then nothing. You get the same with 'freshers' flu' lots of people wringing new germs into one space.
Then there are other factors, think about how small a child's nose and throat are, so a cold can make it harder to breathe than an adult and results in them being more ill.
As a small child my brother caught everything going, I didn't but since puberty my brother has never had more than a cold whereas I have a list of permanent ailments an arm long and multiple hospital visits.