If you believe in sickness, just in general, you will become sick. But that's not the end of it. You'll believe other people are sick too, even if they aren't. And that means that your belief in sickness attempts to...
Either:
The other person is guilty and therefore I am guilty also...
.....OR ....
The other person is innocent and therefore I am innocent also.
There is no other option.
There is no 'they are guilty and I am innocent'. There is no 'they are...
The course speaks from a perspective of Oneness. That means quite literally that you and all other people are ONE person - one mind, one being. It means you SHARE a mind with everyone else. It should be immediately obvious...