"Calling in sick" is one thing. Actually "being sick" is another.
If you're sick, stay home. If what you have is contagious, why would your boss want you at work, anyway? For that matter, why would you?
Going out when you are sick and contagious isn't doing anybody any favors, including yourself. If your boss doesn't understand that, you need to find another boss. Your current one is no good.